Simplify tests part 2

1) Remove some unnecessary fixtures
2) Add EXECUTE_TEST_NO_TEARDOWN shorthand when a fixture exists but has
no teardown.
3) Fix return values in ct_test.c (introduced by an earlier refactoring,
oops)

Note that for parameterized tests, the index (test vector) usually holds all the
customization, and there should be no need for a separate test
fixture. The CTS test is an exception: it demonstrates how to combine
customization with parameterization.

Reviewed-by: Richard Levitte <levitte@openssl.org>