[gcov][test] Add -dumpdir ./

These tests rely on an unintended behavior that when the driver performs both
compilation and linking phases, the .gcno & .gcda files are placed in PWD. The
behavior will be fixed to respect -o (match -ftime-trace, -gsplit-dwarf, and
GCC).

Add -dumpdir ./ so that the tests will work with or without the behavior change,
and make it easy to compare the coverage behavior with GCC.