selftests/sgx: Encpsulate the test enclave creation

Introduce setup_test_encl() so that the enclave creation can be moved to
TEST_F()'s. This is required for a reclaimer test where the heap size needs
to be set large enough to triger the page reclaimer.

Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>
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>
Link: https://lkml.kernel.org/r/bee0ca867a95828a569c1ba2a8e443a44047dc71.1636997631.git.reinette.chatre@intel.com