Tests defs: new variable $top_testsrcdir.

Tests defs: new variable $top_testsrcdir.

* tests/defs.in ($top_testsrcdir): Define unconditionally
to @abs_top_srcdir@.  Use it throughout.
* tests/ar-lib.test: Use `$top_testsrcdir' instead of
`$testsrcdir/..'.
* tests/auxdir.test: Likewise.
* tests/compile.test: Likewise.
* tests/compile2.test: Likewise.
* tests/compile3.test: Likewise.
* tests/compile5.test: Likewise.
* tests/mdate6.test: Likewise.
* tests/mkinst3.test: Likewise.
* tests/multlib.test: Likewise.
* tests/txinfo22.test: Likewise.