testsuite: parallel make fixes.

testsuite: parallel make fixes.

This patch fixes a couple of testsuite bugs exposed with
`MAKE=make\ -jN make check'.
* tests/libtoo10.test: Do not run `clean' in same $MAKE
invocation as `all check'.  Fixes test failure with parallel
NetBSD `make -jN'.
* tests/nobase.test: Be sure to create a directory before
creating files in it.  Fixes test failure with MAKE=`make -jN'
for NetBSD make.  This issue is hidden with parallel GNU make
due to its parallel breadth first update order.
* tests/parallel-tests3.test: Skip if $MAKE contains `-j',
GNU make will use the environment variable $MAKE for recursion
and thus run in parallel even if `$MAKE -j1' was used on the
command line in the test.  Also, after running the test proper,
wait long enough so that background jobs have finished and there
are no open files left when the cleanup code runs.

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