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/9846


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>