tests: remove gexpect from TestAppUserGroup

[lucab]

TestAppUserGroup currently uses gexpect but does not need any
interactivity. This commit switches it away to use golang process
spawning and regexp matching.

Fixes #3560

[lucab]

/cc @evverx @mxinden

[lucab]

This still doesn't improve wrt. #3477 flake, I've still seen local failures.

[lucab]

@s-urbaniak assigning to you for review.

[evverx]

There is another timeout:

=== RUN   TestAppUserGroup
panic: test timed out after 1h0m0s

goroutine 1115 [running]:
panic(0x993d40, 0xc4202b8ff0)
        /home/vagrant/go/src/runtime/panic.go:500 +0x1a1
testing.startAlarm.func1()
        /home/vagrant/go/src/testing/testing.go:918 +0x10b
created by time.goFunc
        /home/vagrant/go/src/time/sleep.go:154 +0x44

goroutine 1 [chan receive, 1 minutes]:
testing.(*T).Run(0xc420088f00, 0xa72c9d, 0x10, 0xad28a0, 0xc420038d01)
        /home/vagrant/go/src/testing/testing.go:647 +0x316
testing.RunTests.func1(0xc420088f00)
        /home/vagrant/go/src/testing/testing.go:793 +0x6d
testing.tRunner(0xc420088f00, 0xc420038e30)
        /home/vagrant/go/src/testing/testing.go:610 +0x81
testing.RunTests(0xad3b98, 0xf52960, 0x8c, 0x8c, 0x1)
        /home/vagrant/go/src/testing/testing.go:799 +0x2f5
testing.(*M).Run(0xc420038ef8, 0x18)
        /home/vagrant/go/src/testing/testing.go:743 +0x85
main.main()
        github.com/coreos/rkt/tests/_test/_testmain.go:364 +0xc6

goroutine 17 [syscall, 59 minutes, locked to thread]:
runtime.goexit()
        /home/vagrant/go/src/runtime/asm_amd64.s:2086 +0x1

goroutine 1122 [syscall]:
syscall.Syscall6(0xf7, 0x1, 0xb5f, 0xc42031b980, 0x1000004, 0x0, 0x0, 0x9d04c0, 0xc42004f670, 0x435b2e)
        /home/vagrant/go/src/syscall/asm_linux_amd64.s:44 +0x5
os.(*Process).blockUntilWaitable(0xc4201a1b90, 0xad3fb8, 0xc42007b080, 0xc4202f0b40)
        /home/vagrant/go/src/os/wait_waitid.go:28 +0xbc
os.(*Process).wait(0xc4201a1b90, 0x0, 0x41150e, 0xc420138328)
        /home/vagrant/go/src/os/exec_unix.go:22 +0xab
os.(*Process).Wait(0xc4201a1b90, 0xc42004f778, 0x0, 0x0)
        /home/vagrant/go/src/os/doc.go:49 +0x2b
os/exec.(*Cmd).Wait(0xc42007b080, 0x0, 0x0)
        /home/vagrant/go/src/os/exec/exec.go:434 +0x6d
os/exec.(*Cmd).Run(0xc42007b080, 0xc42014b7a0, 0xd)
        /home/vagrant/go/src/os/exec/exec.go:279 +0x48
os/exec.(*Cmd).CombinedOutput(0xc42007b080, 0x56, 0xc42000b910, 0xc, 0xf, 0xc42007b080)
        /home/vagrant/go/src/os/exec/exec.go:494 +0x140
github.com/coreos/rkt/tests.runRktAndCheckRegexOutput(0xc420088fc0, 0xc4200a4fc0, 0x21f, 0xa831be, 0x23, 0xc4200a4fc0, 0x21f)
        /home/vagrant/gopath/src/github.com/coreos/rkt/build-rkt-1.23.0+git/gopath/src/github.com/coreos/rkt/tests/rkt_tests.go:458 +0x174
github.com/coreos/rkt/tests.TestAppUserGroup.func1(0xc42004fbd8, 0xc420231301, 0xc42031b800, 0x71, 0xc420088fc0)
        /home/vagrant/gopath/src/github.com/coreos/rkt/build-rkt-1.23.0+git/gopath/src/github.com/coreos/rkt/tests/rkt_run_user_group_test.go:121 +0x404
github.com/coreos/rkt/tests.TestAppUserGroup(0xc420088fc0)
        /home/vagrant/gopath/src/github.com/coreos/rkt/build-rkt-1.23.0+git/gopath/src/github.com/coreos/rkt/tests/rkt_run_user_group_test.go:133 +0x3f7
testing.tRunner(0xc420088fc0, 0xad28a0)
        /home/vagrant/go/src/testing/testing.go:610 +0x81
created by testing.(*T).Run
        /home/vagrant/go/src/testing/testing.go:646 +0x2ec


goroutine 1114 [syscall]:
syscall.Syscall(0x0, 0x4, 0xc4202044de, 0x3922, 0x2b, 0x394d, 0x0)
        /home/vagrant/go/src/syscall/asm_linux_amd64.s:18 +0x5
syscall.read(0x4, 0xc4202044de, 0x3922, 0x3922, 0x2b, 0x0, 0x0)
        /home/vagrant/go/src/syscall/zsyscall_linux_amd64.go:783 +0x55
syscall.Read(0x4, 0xc4202044de, 0x3922, 0x3922, 0x2b, 0x0, 0x0)
        /home/vagrant/go/src/syscall/syscall_unix.go:161 +0x49
os.(*File).read(0xc420138310, 0xc4202044de, 0x3922, 0x3922, 0x2b, 0x0, 0x0)
        /home/vagrant/go/src/os/file_unix.go:228 +0x4f
os.(*File).Read(0xc420138310, 0xc4202044de, 0x3922, 0x3922, 0x2b, 0x0, 0x0)
        /home/vagrant/go/src/os/file.go:101 +0x59
bytes.(*Buffer).ReadFrom(0xc42014b7a0, 0xf56dc0, 0xc420138310, 0xc420384688, 0x40fe01, 0xc420384690)
        /home/vagrant/go/src/bytes/buffer.go:176 +0x155
io.copyBuffer(0xf55180, 0xc42014b7a0, 0xf56dc0, 0xc420138310, 0x0, 0x0, 0x0, 0xc4201453d8, 0x0, 0x0)
        /home/vagrant/go/src/io/io.go:384 +0x323
io.Copy(0xf55180, 0xc42014b7a0, 0xf56dc0, 0xc420138310, 0x98da80, 0xc420145380, 0xc420384790)
        /home/vagrant/go/src/io/io.go:360 +0x68
os/exec.(*Cmd).writerDescriptor.func1(0x98da80, 0xc420145380)
        /home/vagrant/go/src/os/exec/exec.go:253 +0x4d
os/exec.(*Cmd).Start.func1(0xc42007b080, 0xc4202f0b40)
        /home/vagrant/go/src/os/exec/exec.go:370 +0x27
created by os/exec.(*Cmd).Start
        /home/vagrant/go/src/os/exec/exec.go:371 +0x4db

[evverx]

oh, sorry, my bad.
I used GO_TEST_FUNC_ARGS='-count 100 -run TestAppUserGroup'.
I'll try the bash-loop.