testing: make the output of -v more uniform and aligned when using fixed-width fonts

Delete the colon from RUN: for examples, since it's not there for tests.
Add spaces to line up RUN and PASS: lines.

Before:
=== RUN TestCount
--- PASS: TestCount (0.00s)
=== RUN: ExampleFields
--- PASS: ExampleFields (0.00s)

After:
=== RUN   TestCount
--- PASS: TestCount (0.00s)
=== RUN   ExampleFields
--- PASS: ExampleFields (0.00s)

Fixes #10594.

Change-Id: I189c80a5d99101ee72d8c9c3a4639c07e640cbd8
Reviewed-on: https://go-review.googlesource.com/9846Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/testing/example.go

@@ -43,7 +43,7 @@ func RunExamples(matchString func(pat, str string) (bool, error), examples []Int
 
 func runExample(eg InternalExample) (ok bool) {
 	if *chatty {
-		fmt.Printf("=== RUN: %s\n", eg.Name)
+		fmt.Printf("=== RUN   %s\n", eg.Name)
 	}
 
 	// Capture stdout.