build: Skip PK-specific tests if algo is disabled

* configure.ac: Define AM_CONDITIONALs for USE_DSA, USE_RSA,
  USE_ELGAMAL, USE_ECC so Makefiles can depend on them.
* tests/Makefile.am: Skip tests that test only one public key algorithm
  if that algorithm is disabled.

Skip building and running tests that are specific for a public key
algorithm if that algorithm was disabled using the
--enable-pubkey-ciphers configure option.

• GnuPG-bug-id: T6048
• Signed-off-by: Clemens Lang <cllang@redhat.com>