selftests/sgx: Do not attempt enclave build without valid enclave

It is not possible to build an enclave if it was not possible to load
the binary from which it should be constructed. Do not attempt
to make further progress but instead return with failure. A
"return false" from setup_test_encl() is expected to trip an
ASSERT_TRUE() and abort the rest of the test.

Fixes: 1b35eb71 ("selftests/sgx: Encpsulate the test enclave creation")
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Acked-by: Dave Hansen <dave.hansen@linux.intel.com>
Acked-by: Shuah Khan <skhan@linuxfoundation.org>
Link: https://lkml.kernel.org/r/e3778c77f95e6dca348c732b12f155051d2899b4.1644355600.git.reinette.chatre@intel.com