Rollup merge of #42219 - pwoolcoc:add-allow-fail-to-libtest, r=GuillaumeGomez

add `allow_fail` test attribute

This change allows the user to add an `#[allow_fail]` attribute to
tests that will cause the test to compile & run, but if the test fails
it will not cause the entire test run to fail. The test output will
show the failure, but in yellow instead of red, and also indicate that
it was an allowed failure.

Here is an example of the output: http://imgur.com/a/wt7ga

0 files changed, 0 insertions, 0 deletions