commit 5283ea9b changed the
semantics of the 'expire_offset' field in the test case struct
so that instead of being an absolute timestamp, it was a delta
relative to the current time. This broke the test cases which
were testing expiry of certificates, by putting the expiry
time into the future, instead of in the past.

Fix this by changing the expiry values to be negative, so that
the delta goes into the past again.

* virnettlscontexttest.c: Fix expiry tests

* tests/virnettlscontexttest.c (testTLSLoadKey): Report errors.

Even though gnutls is a hard-req for libvirt, and gnutls depends
on libtasn1, that does not mean that you have to have the libtasn1
development files installed.  Skip the test rather than failing
compilation in that case.

With newer gcc, the test consumed too much stack space.  Move
things to static storage to fix that.

* configure.ac (AC_CHECK_HEADERS): Check for libtasn1.h.
(HAVE_LIBTASN1): New automake conditional.
* tests/Makefile.am (virnettlsconvirnettlscontexttest_SOURCES)
(virnettlscontexttest_LDADD): Allow compilation without libtasn1.
* tests/virnettlscontexttest.c: Skip test if headers not present.
(struct testTLSCertReq): Alter time members.
(testTLSGenerateCert): Reflect the change.
(mymain): Reduce stack usage.

This test case checks certification validation rules for

 - Basic constraints
 - Key purpose
 - Key usage
 - Start/expiry times

It checks initial context creation sanity checks, and live
session validation