selftests/sgx: Move setup_test_encl() to each TEST_F()

Create the test enclave inside each TEST_F(), instead of FIXTURE_SETUP(),
so that the heap size can be defined per test.

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/70ca264535d2ca0dc8dcaf2281e7d6965f8d4a24.1636997631.git.reinette.chatre@intel.com