go: honor [go-test].force_race and the test_race/test_msan/test_asan fields

[aegis-cipherowl]

prepare_go_test_binary builds its GoBuildOptionsFromTargetRequest without for_tests=True:

go_extract_build_options_from_target(
    GoBuildOptionsFromTargetRequest(request.field_set.address), **implicitly()
),

go_extract_build_options_from_target only populates test_target_fields when that flag is set:

if request.for_tests and GoTestBuildOptionsFieldSet.is_applicable(target):
    test_target_fields = GoTestBuildOptionsFieldSet.create(target)

and test_target_fields gates both the [go-test].force_race branch and the go_package(test_race=...) field (likewise test_msan / test_asan).

go_package also does not carry the non-test race / cgo_enabled fields, so GoBuildOptionsFieldSet does not apply to it either and that fallback misses as well. The net effect is that the only field the resolution chain can still reach is race on the owning go_mod, so both documented ways to enable the race detector for a test are silently ignored — tests keep passing while no longer detecting races.

Introduced in #17825, which split test-binary production from running it.

The existing coverage in build_opts_test.py passes for_tests=True explicitly, which is why it never caught the buggy caller.

Fix

Pass for_tests=True. One line.

Tests

Three tests in test_test.py, exercising [go-test].force_race, go_package(test_race=True), and a default-off control so the probe cannot pass for the wrong reason.

They use a real unsynchronised write rather than a //go:build race file. That was my first attempt, and it does not work: package analysis passes CGO_ENABLED and EXTRA_BUILD_TAGS but never the race build tag, so //go:build race files are not selected even when -race does reach the compiler. That looks like a separate gap and I have deliberately left it alone here — happy to file it separately if it is not already known.

Verified red/green on macOS/arm64:

	before	after
test_race_detector_enabled_by_force_race_option	fails (race unreported)	passes
test_race_detector_enabled_by_test_race_field	fails (race unreported)	passes
test_race_detector_disabled_by_default	passes	passes

The tests skip on platforms where race_detector_supported() is false.

Note: build_opts_test.py::test_runtime_check_enable_fields_work_as_expected[msan|asan] and test_test.py::test_external_test_with_use_coverage already fail on my machine on an unmodified main, so they are unrelated to this change.