selftests/sgx: Use a statically generated 3072-bit RSA key

Use a statically generated key for signing the enclave, because
generating keys on the fly can eat the kernel entropy pool. Another
good reason for doing this is predictable builds. The RSA has been
arbitrarily selected. It's contents do not matter.

This also makes the selftest execute a lot quicker instead of the delay
that it had before (because of slow key generation).

 [ bp: Disambiguate "static key" which means something else in the
   kernel, fix typos. ]

Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: linux-kselftest@vger.kernel.org
Link: https://lkml.kernel.org/r/20201118170640.39629-1-jarkko@kernel.org