Add --with-timeoutfactor=NUM to specify TIMEOUTFACTOR

On Ice Lake and Tiger Lake laptops, some test programs timeout when there
are 3 "make check -j8" runs in parallel.  Add --with-timeoutfactor=NUM to
specify an integer to scale the timeout of test programs, which can be
overriden by TIMEOUTFACTOR environment variable.

Reviewed-by: Florian Weimer <fweimer@redhat.com>