runtime/trace: add stack tests for GOMAXPROCS

and reorganize test log messages for stack dumps
for easier debugging.

The error log will be formatted like the following:

	trace_stack_test.go:282: Did not match event GoCreate with stack
		 runtime/trace_test.TestTraceSymbolize	 :39
		 testing.tRunner			 :0

		Seen 30 events of the type
		Offset 1890
		 runtime/trace_test.TestTraceSymbolize	/go/src/runtime/trace/trace_stack_test.go:30
		 testing.tRunner			/go/src/testing/testing.go:777
		Offset 1899
		 runtime/trace_test.TestTraceSymbolize	/go/src/runtime/trace/trace_stack_test.go:30
		 testing.tRunner			/go/src/testing/testing.go:777
		 ...

Change-Id: I0468de04507d6ae38ba84d99d13f7bf592e8d115
Reviewed-on: https://go-review.googlesource.com/92916


Reviewed-by: Heschi Kreinick <heschi@google.com>
Run-TryBot: Hyang-Ah Hana Kim <hyangah@gmail.com>