build: Check for the IBM TSS tools to run the tpm2d tests.

* configure.ac (TEST_LIBTSS): Make that conditional depend on the
detection of tssstartup.

While the tpm2d daemon can use either the Intel TSS or the IBM TSS,
the test code (specifically, the start_sw_tpm.sh script) requires
some tools from the IBM TSS (tssstartup and tsspowerup).

If a software TPM is detected at configure time but the IBM TSS
tools are absent (e.g. because only the Intel TSS is available on
the system), make check will attempt to run the tpm2d tests and
those will fail when the start_sw_tpm.sh script is launched.

This patch makes running the tpm2d tests dependent not only on
the detection of a software TPM, but also on the detection of
tssstartup (it is probably safe to assume that if tssstartup is
present, then tsspowerup is available as well).

• Signed-off-by: Damien Goutte-Gattat <dgouttegattat@incenp.org>