maintcheck: look for problematic names of testcases

maintcheck: look for problematic names of testcases

The configure.in stub created by default by `tests/defs' obtains
the first argument of AC_INIT from the test name, and this can
cause some supported autoconf versions to fail with a spurious
error if that test name contains the name of an m4 or m4sugar
builtin or macro (e.g., `defn' or `m4_undefine').

See for example the bug fixed by commit v1.11-287-g1325a8a.

This change add a maintainer check that warns about test names
which are possibly problematic in this regard.

* Makefile.am (sc_test_names): New maintainer-check target.
(syntax_check_rules): Add it.
(m4_builtins): New helper variable.
(TESTS): Updated according to the following renamings.
* tests/include.test: Renamed ...
* tests/hdr-vars-defined-once.test: ... to this.
* tests/sinclude.test: Renamed ...
* tests/m4-inclusion.test: ... to this, and simplified
accordingly.
* tests/include2.test: Renamed ...
* tests/dist-included-parent-dir.test: ... to this, for
consistency.