proposal: testing: add a ParallelCount func that reports the go test -p flag value #50719

Summary:
Add a helper to the testing package that exposes the value of the go test -p flag. This is similar to testing.Verbose() or testing.Short() functions that expose the values of other flags.

Problem:
By default, go test runs with with the -p flag defaulting to GOMAXPROCS, normally the number of CPUs available. This results in tests within separate packages running in parallel, which is great, I get fast test runs.

However, occasionally I am forced to write tests that are racy with concurrent tests. For example when two tests in separate packages each call t.Setenv() to set the same env var.

The workaround is to run go test -p=1 to force the tests to run sequentially. But that drastically slows down the tests, plus most of the time I actually want tests to run concurrently to catch unexpected races etc.

Often there's just two or three tests out of hundreds within a package that need to be run sequentially. So it's not feasible to use the -run flag to exclude all the other tests... instead I end up with something like this in my code:

if os.GetEnv("TESTS_ARE_BEING_RUN_SERIALLY") != "true" {
	t.Skip("Skipping " + t.Name() + " as it is racy when run concurrently with other tests due to using t.Setenv().")
}

which I then have to invoke in CI via something like:

- go test ./...
- TESTS_ARE_BEING_RUN_SERIALLY="true" go test -p=1 -run <regex of tests that are racy> ./...

The TESTS_ARE_BEING_RUN_SERIALLY env var is clunky, and certainly confusing if you're not familiar with the code.

So it'd be great if the testing package had a helper similar to testing.Verbose() or testing.Short() functions that exposed the value of the -p flag:

func ParrallelCount() int {
	// return value of `-p` flag to `go test`
}

which I would use in my test like this:

if testing.ParrallelCount() > 1 {
	t.Skip("Skipping " + t.Name() + " as it is racy when run concurrently with other tests due to using t.Setenv().")
}

Notes:

• For my use case, a bool would suffice, I don't actually care how many, just whether it's more than 1 or not. But I suspect there may be use cases where knowing the actual number of parallel test runners would be useful.
• The parallel count of running test runners can change dynamically--ie, if some packages finish their tests before others. For my purposes, I don't want the "real time" count, but rather the max-allowed which is what -p flag controls.
• The scope of this proposed helper is confusing because the parallel count of running tests is affected by both go test -p=n and t.Parallel() helper. Since t.Parallel() does nothing if -p=1 (at least that's my understanding), then for my purposes, I only care about the -p flag setting.
• The naming is confusing... if I saw a testing.ParallelCount() helper, it's easy to think it's related to the t.Parallel() helper... But I struggled to come up with a more appropriate name. Perhaps the scoping of a pkg level helper (testing) vs a test-level helper (t) is enough disambiguation?