tests/tpm: Display if swtpm is not found or --tpm2 not supported

tests/tpm: Display if swtpm is not found or --tpm2 not supported

If swtpm is not found in $PATH or --tpm2 isn't supported, we display
this in the test log. We cannot mark the test as skipped due to a bug in
certain versions of the gtester environment that interprets a skipped test
as failure.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Stefan Berger <stefanb@linux.vnet.ibm.com>
Signed-off-by: Stefan Berger <stefanb@linux.vnet.ibm.com>