Add gcov MC/DC tests for GDC

Add gcov MC/DC tests for GDC

This is a mostly straight port from the gcov-19.c tests from the C test
suite. The only notable differences from C to D are that D flips the
true/false outcomes for loop headers, and the D front end ties loop and
ternary conditions to slightly different locus.

The test for >64 conditions warning is disabled as it either needs
support from the testing framework or a something similar to #pragma GCC
diagnostic push to not cause a test failure from detecting a warning.

gcc/testsuite/ChangeLog:

	* gdc.dg/gcov.exp: New test.
	* gdc.dg/gcov1.d: New test.