x/build/cmd/coordinator: compile error in `os/os_test.go` not detected by `misc-compile` TryBots

[bcmills]

https://go.dev/cl/517755 caused a compilation error on the plan9 builders (https://build.golang.org/log/2613479c07e3aaf02e75f15f00b8d1c01c9ec217):

# os_test [os.test]
os/os_test.go:3297:70: too many arguments in call to syscall.Open
	have (string, number, number)
	want (string, int)
os/os_test.go:3302:55: too many arguments in call to syscall.Open
	have (string, number, number)
	want (string, int)
FAIL	os [build failed]

I didn't catch that during code review because the CL had a passing TryBot run, and based on #20119 I assumed that the misc-compile TryBots would have caught any compile error in the test.

It appears that either the fix for #20119 was incomplete, or something has regressed since then to cause the misc-compile TryBots to no longer catch compile errors in tests.

(CC @dmitshur @aclements @golang/release)

[bcmills]

I suspect this regressed in https://go.dev/cl/464955 (for #58163; attn @mknyszek).

[mknyszek]

Thanks for flagging this, on it.

[mknyszek]

OK yeah, got it. I suspect this should be an easy fix.

[gopherbot]

Change https://go.dev/cl/518255 mentions this issue: os: define TestIssue60181 only on Unix platforms

[dmitshur]

For the record, this is is being caught in LUCI as intended, see the misccompile column at https://ci.chromium.org/p/golang/g/go-gotip-ci/console. So this issue only affects x/build/cmd/coordinator.

[gopherbot]

Change https://go.dev/cl/518295 mentions this issue: cmd/coordinator: build test packages for misc-compile builders