selftests/sgx: Migrate to kselftest harness

Migrate to kselftest harness. Use a fixture test with enclave initialized
and de-initialized for each of the existing three tests, in other words:

1. One FIXTURE() for managing the enclave life-cycle.
2. Three TEST_F()'s, one for each test case.

Dump lines of /proc/self/maps matching "sgx" in FIXTURE_SETUP() as this
can be very useful debugging information later on.

Amended commit log:
This migration changes the output of this test. Instead of skipping
the tests if open /dev/sgx_enclave fails, it will run all the tests
and report failures on all of them.
Shuah Khan <skhan@linuxfoundation.org>

Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>