tpm2d: add tests for the tpm2daemon

* configure.ac: Detect TPM emulator and enable tests.
* tests/tpm2dtests/: New test suite.
* tests/Makefile.am: Run tests.

Add a set of tests that exercise tpm2daemon handling of keys and
verify compatibility with non-tpm based keys.

Running this test infrastructure requires a tpm emulator, which is
tested for during configuration. If an emulator is not found, the
tests won't be run since they require the presence of a TPM (although
the TPM handling code will still be built).

• Fixed tests/Makefile.am for make distcheck.

• Signed-off-by: Werner Koch <wk@gnupg.org>