Summary:
With the Makefile now updated to correctly update all .o files, this
should fix the issues recompiling stringappend_test. This should also fix the
"segmentation-fault" that we were getting earlier. Now, stringappend_test should
be clean, and I have added it back to the unit-tests. Also made some minor updates
to the tests themselves.

Test Plan:
1. make clean; make stringappend_test -j 32	(will test it by itself)
2. make clean; make all check -j 32		(to run all unit tests)
3. make clean; make release			(test in release mode)
4. valgrind ./stringappend_test 		(valgrind tests)

Reviewers: haobo, jpaton, dhruba

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D11505