testing: quote -test.v=test2json output when logging it
The -test.v=test2json flag causes the testing package to inject extra
control characters in the output to allow the JSON parser to more
gracefully handle extraneous writes to os.Stdout and/or os.Stderr in
the package under test (see CL 443596). However, it doesn't filter out
those control characters because almost no real-world tests will
output them.
It turns out that testing.TestFlag is one of the rare tests that does
output those control characters, because it tests the
-test.v=test2json flag itself.