test: Allow running tests multiple times

Some tests can have race conditions which are hard to detect on a single
one. Add a way to run tests more than once, to help with this.

Each individual test is run the requested number of times before moving
to the next test. If any runs failed, a message is shown.

This is most useful when running a single test, since running all tests
multiple times can take a while.

Signed-off-by: Simon Glass <sjg@chromium.org>