Fix testsuite program wrapper for whitespace in `pwd`.

Fix testsuite program wrapper for whitespace in `pwd`.

The problem here is that the usual mantra is that command
variables can contain arguments, thus we cannot just escape
$AUTOCONF, $AUTOM4TE etc.  The compromise is to put the
$top_builddir/tests directory early in $PATH, so that the
wrappers are found by their plain name.

* tests/wrapper.as: Put $testdir early in $PATH.
(AUTOCONF, AUTOHEADER, AUTOM4TE): Set to plain command names.