Fix error in generation of parallel tests.

Fix error in generation of parallel tests.

* tests/defs.in ($am_skip_defs): New variable, to be used when
./defs must be sourced multiple times.  If set, unset it and
only define $srcdir; otherwise, also go through the rest of
the script.
($am_defs_included): Remove, no more needed.
* tests/Makefile.am ($(parallel_tests)): Update accordingly,
using only $srcdir from defs.
Fixes potential test failures of tests that use $required.

Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>