build: use run.go for running tests

Also, tweak run.go to use no more than 2x the
number of CPUs, and only one on ARM.

53.85u 13.33s 53.69r 	 ./run
50.68u 12.13s 18.85r 	 go run run.go

Fixes #2833.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/5754047

src/run.bash

@@ -105,7 +105,7 @@ $BROKEN ||
 ) || exit $?
 
 (xcd ../test
-./run
+time go run run.go
 ) || exit $?
 
 echo

src/run.bat

@@ -36,7 +36,14 @@ go test sync -short -timeout=120s -cpu=10
 if errorlevel 1 goto fail
 echo.
 
-:: TODO: The other tests in run.bash, especially $GOROOT/test/run.
+:: TODO: The other tests in run.bash.
+
+echo # test
+cd test
+go run run.go
+cd ..
+if errorlevel 1 goto fail
+echo.
 
 echo ALL TESTS PASSED
 goto end

test/run.go

@@ -30,7 +30,7 @@ import (
 
 var (
 	verbose     = flag.Bool("v", false, "verbose. if set, parallelism is set to 1.")
-	numParallel = flag.Int("n", 8, "number of parallel tests to run")
+	numParallel = flag.Int("n", 2*runtime.NumCPU(), "number of parallel tests to run")
 	summary     = flag.Bool("summary", false, "show summary of results")
 	showSkips   = flag.Bool("show_skips", false, "show skipped tests")
 )
@@ -60,7 +60,10 @@ const maxTests = 5000
 
 func main() {
 	flag.Parse()
-	if *verbose {
+	
+	// Disable parallelism if printing, or if running on
+	// (presumably underpowered) arm systems.
+	if *verbose || runtime.GOARCH == "arm" {
 		*numParallel = 1
 	}