Make is:tentative match tentative directories

[gsnedders]

Per https://web-platform-tests.org/writing-tests/file-names.html, tentative can be set at a directory level via the directory being called "tentative".

https://wpt.fyi/results/css/css-borders?label=master&label=experimental&aligned&q=is%3Atentative notably misses the entire "tentative" directory.

[gsnedders]

The go_test regression is:

# github.com/web-platform-tests/wpt.fyi/api/query/cache/index [github.com/web-platform-tests/wpt.fyi/api/query/cache/index.test]
api/query/cache/index/index_filter_test.go:1073:8: duplicate field name Test in struct literal
api/query/cache/index/index_filter_test.go:1074:16: duplicate field name LegacyStatus in struct literal
api/query/cache/index/index_filter_test.go:1082:8: duplicate field name Test in struct literal
api/query/cache/index/index_filter_test.go:1083:16: duplicate field name LegacyStatus in struct literal
FAIL	github.com/web-platform-tests/wpt.fyi/api/query/cache/index [build failed]

I seem to have messed up committing my code after gofmt and broken it after I ran the tests, whoops.

And then web_components_test fails because of #4046.