RSASSA-PSS with SHA-512 is slightly too large for 1024-bit RSA. One
should not be using 1024-bit RSA, but it's common enough for tests
(including our own in runner before they were regenerated), that we
should probably do the size check and avoid unnecessary turbulence to
everyone else's test setups.

Change-Id: If0c7e401d7d05404755cba4cbff76de3bc65c138
Reviewed-on: https://boringssl-review.googlesource.com/8746
Reviewed-by: Steven Valdez <svaldez@google.com>
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>