TESTS: simple CA to generate certificates for test
To avoid issue with certificate lifetimes a simple OpenSSL based CA is
used to generate certificates for tests.
To make management easy all related data is kept in
src/tests/test_CA. Since some header files will be generated the
generation of the needed files is added to BUILT_SOURCES as other
generated code.
Related to https://pagure.io/SSSD/sssd/issue/3436
Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>