...by placing cert8.db (and automatically created key3.db, secmod.db) in
workdir/CppunitTest/xmlsecurity_signing.test.user/, which gets recreated afresh
on every test run.  (From solenv/gbuild/CppunitTest.mk, that path should be
available in the test as the value of the UserInstallation bootstrap var, but
then test::BootstrapFixtureBase::setUp in
unotest/source/cpp/bootstrapfixturebase.cxx explicitly resets UserInstallation
to someplace else.)

Change-Id: Iebd2956134ec2226b28024e7c26e02b642a1e6b1