Rework TestGenerator to add file targets

BaseTarget-derived targets are now added to TestGenerator.targets in
initialization. This reduces repeated code in generate_xxx_tests.py
scripts which use this framework.

Signed-off-by: Werner Lewis <werner.lewis@arm.com>
diff --git a/tests/scripts/generate_psa_tests.py b/tests/scripts/generate_psa_tests.py
index ebec2ee..1f516c9 100755
--- a/tests/scripts/generate_psa_tests.py
+++ b/tests/scripts/generate_psa_tests.py
@@ -898,7 +898,7 @@
     """Test generator subclass including PSA targets and info."""
     # Note that targets whose names contain 'test_format' have their content
     # validated by `abi_check.py`.
-    TARGETS = {
+    targets = {
         'test_suite_psa_crypto_generate_key.generated':
         lambda info: KeyGenerate(info).test_cases_for_key_generation(),
         'test_suite_psa_crypto_not_supported.generated':