Make Test;More;ok and nok use a PMC argument

[flh]

The first argument of Test;More;ok and Test;More;nok is currently an integer.

When we call "ok($P0)", the VTABLE method "get_integer" gets called on $P0, while we would expect the "get_bool" method to be called.

The attached patch:

• turns the "passed" argument of Test;More;ok and Test;More;nok into a PMC, on which we call istrue (or isfalse for nok),
• implements the "get_bool" in Env, to prevent a failure in t/pmc/env.t,
• adds tests in t/library/test_more.t, which pass with the new behaviour, but fail with the current one,
• tests the Test;More;nok (copied from ok), which wasn't tested before.