Added basic unit test for llvm-cov.

This test compares the output of llvm-cov against a coverage file
generated by gcov.
Revert "Added basic unit test for llvm-cov."

This reverts commit 9cacd131c22b888303cb88e9a3235b2d7b2f19a1.
Revert "llvm-cov: Added command-line option to change dir."

This reverts commit d8acf0078cf363252727acff00f85ae8074f95b3.