test: document specifying individual test files as operands
test: document specifying individual test files as operands

The current command will run this entire set of tests, which takes a
noticeable amount of time. Contributors may wish to run only a subset of
these tests to save time/compute (e.g. when iterating on a CL that
failed tests in that subset). Listing file(s) as operands to the command
will run only those tests.

Change-Id: I1874c43681a594190bc40b61cee0b8d321be73f8
Reviewed-on: https://go-review.googlesource.com/c/go/+/242997
Reviewed-by: Daniel Martí <29bf28b42a938ace12a5819da4ed4ba03d82f315@mvdan.cc>

golang/go,nawawi/go,cloudflare/go,golang/go,cloudflare/go,cloudflare/go,cloudflare/go,golang/go,CAFxX/go,cloudflare/go,nawawi/go,CAFxX/go,CAFxX/go,CAFxX/go,golang/go,nawawi/go,nawawi/go,CAFxX/go,CAFxX/go,nawawi/go,golang/go,golang/go,nawawi/go,golang/go,nawawi/go,CAFxX/go,cloudflare/go,CAFxX/go,cloudflare/go,nawawi/go,golang/go,cloudflare/go