Capture googletest output once after shrinking only

[zombiecalypse]

Currently gtest will log a failure for each EXPECT_* in the test, but with shrinking this leads to each shrinking step to have it's own failure.

I learned that wrapping testing::ScopedFakeTestPartResultReporter from <gunit/gunit-spi.h> allows to redirect all failure messages into a collection so that it will only be reported when replayed.