Use gtest functions for comparing hash strings

This prints the mismatched strings if the test failed,
simplifying managing and updating the test suite.
Add a test with sliced encoding

This serves as a simple test of the thread library.
Merge pull request #390 from mstorsjo/gtest-strcmp

Use gtest functions for comparing hash strings
Merge pull request #391 from mstorsjo/threaded-test

Add a test with sliced encoding
Merge remote-tracking branch 'upstream/master' into travis.android