tests: Convert %CONFIG_FLAGS to get_config()

tests: Convert %CONFIG_FLAGS to get_config()

* tests/config-flags.pm.W32: Create a predefined Windows file.
* Makefile.am (test_FILES): Add it to the distribution.
* build_w32.bat: Install tests/config-flags.pm if not existing.
* tests/run_make_tests.pl (get_config): Create new function.
* tests/scripts/features/archives: Call get_config() rather than
using %CONFIG_FLAGS directly.
* tests/scripts/features/load: Ditto.
* tests/scripts/features/loadapi: Ditto.
* tests/scripts/functions/wildcard: Ditto.