Add test for RefBase etc.

Add some basic tests for RefBase, as well as a more ambitious memory
ordering test.

Add a README.txt with instructions to run the tests.

Comment out a couple of BlobCache tests that failed consistently and
appeared to be incorrect.  With that fix, I managed to run
libutils_tests successfully on device.

Bug: 28705989
Change-Id: I8ad29995097a149a0cc38615d6ed37117ec6cb5c