libphobos: Run unittest tests with dg-runtest.

Use `dg-runtest' test driver rather than `dg-test' to run the libphobos
unittest testsuite, same as all other libphobos tests.  This prevents
the tests from being ran multiple times when parallelized.

Set `libphobos_test_name' as well so that all tests get a unique name.

libphobos/ChangeLog:

	* testsuite/libphobos.unittest/unittest.exp: Use `dg-runtest' rather
	than `dg-test'.  Set `libphobos_test_name'.