errata: add ERRATA_FORCE

Provide an easy way for test runners to run all tests, even those
guarded by an erratum. Setting ERRATA_FORCE forces errata() and
errata_relaxed() to always return true. ./run_tests.sh -a is
also updated to set it.

Signed-off-by: Andrew Jones <drjones@redhat.com>
Reviewed-by: Laurent Vivier <lvivier@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>