cmd/compile/internal/test: clean up TestPGOIntendedInlining

cmd/compile/internal/test: clean up TestPGOIntendedInlining

The most important change here is to log output from the child, making
it easier to diagnose problems when the child 'go test' fails.

We can also eliminate the cmd.Wait goroutine by using an os.Pipe, whose
reader will return io.EOF when the child exits.

For #55022.

Change-Id: I1573ea444407d545bdca8525c9ff7b0a2baebf5e
Reviewed-on: https://go-review.googlesource.com/c/go/+/446300
Run-TryBot: Michael Pratt <mpratt@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>