Have rust_test put its compilation outputs in a subdirectory

[scentini]

This is supposed to fix the long standing Windows flakiness as described in #1427 (comment)

Initially I thought it's an issue with rustdoc_test, however the actual issue is that rust_binary and its rust_test have the same crate name and generate the same intermediary .o files. For sandboxed builds this is not an issue, as the actions are isolated, however, we have a race condition in non-sandboxed builds (Windows):

Let's consider the following build:

bazel build --spawn_strategy=standalone \
    //test/unit/rustdoc:bin_with_transitive_proc_macro \
    //test/unit/rustdoc:bin_with_transitive_proc_macro_test

The bin_with_transitive_proc_macro compile action will create the following intermediate file: bazel-out/k8-fastbuild/bin/test/unit/rustdoc/bin_with_transitive_proc_macro.bin_with_transitive_proc_macro.573369dc-cgu.2.rcgu.o, as will the bin_with_transitive_proc_macro_test action. These two files differ slightly (as the second action is for a test), namely the bin_with_transitive_proc_macro output has the following symbols:

0000000000000000 T main
0000000000000000 t _ZN30bin_with_transitive_proc_macro4main17hfc292cc42314e131E
                 U _ZN3std2rt10lang_start17h1dbc829c47cd61d9E

while the bin_with_transitive_proc_macro_test .o output looks like this:

0000000000000000 T main
0000000000000000 t _ZN30bin_with_transitive_proc_macro4main17h28726504dc060f8dE
                 U _ZN3std2rt10lang_start17h1dbc829c47cd61d9E
                 U _ZN4test16test_main_static17h37e3d88407f5b40fE

Now, when the two actions run in parallel, it can happen that bin_with_transitive_proc_macro creates the output, and bin_with_transitive_proc_macro_test overwrites it. Then, at linking time for bin_with_transitive_proc_macro, rustc will complain:

note: ld.lld: error: undefined symbol: test::test_main_static::h37e3d88407f5b40f

This is because bin_with_transitive_proc_macro is not a test and as such is not linked against libtest.

This PR fixes the issue by directing the compilation outputs of rust_test to be under a test-{hash} directory.