cmd/go: stream test output if parallelism is set to 1.

"go test -v" buffers output if more than one package is
being tested to avoid mixing the outputs from multiple
tests running in parallel. It currently enables streaming
if there's only a single package under test.

It is ok to stream output from multiple tests if we know
that they're not going to be running in parallel.

To see the difference: go test -v -p=1 runtime fmt -short

Change-Id: Idc24575c899eac30d553e0bf52b86f90e189392d
Reviewed-on: https://go-review.googlesource.com/4153Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/test.go

@@ -346,11 +346,11 @@ func runTest(cmd *Command, args []string) {
 	// been given on the command line (implicit current directory)
 	// or when benchmarking.
 	// Also stream if we're showing output anyway with a
-	// single package under test.  In that case, streaming the
-	// output produces the same result as not streaming,
-	// just more immediately.
+	// single package under test or if parallelism is set to 1.
+	// In these cases, streaming the output produces the same result
+	// as not streaming, just more immediately.
 	testStreamOutput = len(pkgArgs) == 0 || testBench ||
-		(len(pkgs) <= 1 && testShowPass)
+		(testShowPass && (len(pkgs) == 1 || buildP == 1))
 
 	var b builder
 	b.init()