Tests defs: move static definitions in a new file `defs-static'.

Tests defs: move static definitions in a new file `defs-static'.

The new file is meant to be idempotent w.r.t. mutiple inclusions.

* tests/defs.in: Removed, its contents split among ...
* tests/defs-static.in: ... this new file ...
* tests/defs: ... and this new file, including the former.
* configure.ac (AC_CONFIG_FILES): Remove `tests/defs', add
`tests/defs-static'.
(AC_CONFIG_LINKS): Add `tests/defs'.
* tests/Makefile.am ($(parallel_tests)): Update.
($(instspc_tests)): Likewise.
* tests/.gitignore: Update.