selftests/sgx: Page removal stress test

Create enclave with additional heap that consumes all physical SGX
memory and then remove it.

Depending on the available SGX memory this test could take a
significant time to run (several minutes) as it (1) creates the
enclave, (2) changes the type of every page to be trimmed,
(3) enters the enclave once per page to run EACCEPT, before
(4) the pages are finally removed.

Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Acked-by: Jarkko Sakkinen <jarkko@kernel.org>
Link: https://lkml.kernel.org/r/e7c6aa2ab30cb1c41e52b776958409c06970d168.1652137848.git.reinette.chatre@intel.com