tests: minor optimizations/simplifications in some aclocal tests

tests: minor optimizations/simplifications in some aclocal tests

* acloca11.test: Instead of running ./configure and then grepping
the file it has produced, directly grep configure.  Add trailing
`:' command.
* acloca12.test: Likewise.
* acloca10.test: Likewise.  Also, remove the autom4te caches
by hand, instead of sleeping to ensure they get outdated and
ignored.
* aclocal9.test: Likewise.
* acloca18.test: Likewise.  Also, do not remove file `foo',
which is not being created anymore.
* acloca20.test: The test makes sense only if the system support
real symlinks, so skip it if this is not the case.  This allows
for various simplifications.