TFTF: ARMv8.3-PAuth test code enhancements

This patch provides the following feature and makes
modification listed below:
- ARMv8.3-PAuth tests now check for all keys being in use
 (e.g. APIAKey when the test suite is built with
 `ENABLE_PAUTH=1` option) and program new key values otherwise.

Signed-off-by: Alexei Fedorov <Alexei.Fedorov@arm.com>
Change-Id: Ifa4a288274822029da585073563c68a1434f5de7