Make the default testharness.js timeout longer than the LayoutTests timeout

With the previous defaults, tests could timeout before the LayoutTests
timeout, which would be reported as a failure. Making the timeout very
long should fix tests that are flaky with the 2 second timeout and will
make the SlowTests mechanism work for testharness.js tests, should the
need arise.

BUG=355489

Review URL: https://codereview.chromium.org/213283002

git-svn-id: svn://svn.chromium.org/blink/trunk@170136 bbb929c8-8fbe-4397-9dbb-9b2b20218538