Remove unused SapphireTest::is_running_test check and any other references to it

[robbieaverill]

The test suite at the moment is passing without throwing this exception: #141 (comment)

The fact that it's not being executed at the moment is not having an adverse effect on the test suite, so we should just remove this instead.

Also check for other references to is_running_test in the code and remove as necessary.

[raissanorth]

SapphireTest::is_running_test is not referenced anywhere else. See b448bfc

[robbieaverill]

Thanks!