glcpp: Add --valgrind option to the glcpp-test utility

glcpp: Add --valgrind option to the glcpp-test utility

The common case for this test suite is to quickly test that everything
returns the correct results. In this case, the second run of the test
suite under valgrind was just annoying, (and the user would often
interrupt it).

Now, do what is wanted in the common case by default (just run the
test suite), and require a run with "glcpp-test --valgrind" in order
to test with valgrind.