arch-run: provide errata from run env

Allow the run script environment to provide ERRATA_* variables
to the unit tests. These variables will augment/override variables
in any provided unit test environ (the file specified with $ENV)
or trigger the creation of a temporary environ file, which contains
them.

Now something like

 ERRATA_abcd01234567=y ./run_tests.sh

may be run in order to inform all unit tests that they may execute
all subtests guarded by ERRATA_abcd01234567.

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