cmd/internal/testdir: test constraint parsing fails when test sources are `gofmt`'d

[bcmills]

In https://go.dev/cl/522495 I attempted to fix a test failure by editing test/fixedbugs/issue10607.go.

My editor invoked gopls to format the file on save, which added a //go:build line to the file and added a newline after the // +build line. I assumed that would be ok, because nothing about the test is specific to the parser.

However, that caused the action parsing in testdir_test.run to fail, because it expects the first blank line to follow the “action” comment, not the //go:build line.

[gopherbot]

Change https://go.dev/cl/522635 mentions this issue: cmd/internal/testdir: parse past gofmt'd //go:build lines