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
diff --git a/libutils/tests/Android.bp b/libutils/tests/Android.bp
index 9e2fd5f..ec6b67f 100644
--- a/libutils/tests/Android.bp
+++ b/libutils/tests/Android.bp
@@ -24,6 +24,7 @@
         "BitSet_test.cpp",
         "Looper_test.cpp",
         "LruCache_test.cpp",
+        "RefBase_test.cpp",
         "String8_test.cpp",
         "StrongPointer_test.cpp",
         "SystemClock_test.cpp",