Build: Decouple S/NS test control in build configs

Regression tests for both Secure and Non-Secure are currently controlled
by the same boolean seed param "test_regression", which makes the tests
can only be enabled/disable at the same time.

Now that it is needed by some platforms such as Corstone1000 to only
enable Secure tests without Non-Secure tests, the test control needs to
be decoupled.

This patch converts "test_regression" from boolean to string to indicate
which tests should be enabled.

Signed-off-by: Xinyu Zhang <xinyu.zhang@arm.com>
Change-Id: I1e73dc5d7bca7239fcdd2050bb5e58389a79ad95
4 files changed