Make some `confh*.test' tests more "semantic" (plus tweakings).

Make some `confh*.test' tests more "semantic" (plus tweakings).

* tests/confh.test: Run "autoconf", "configure" and "make check",
instead of munging/grepping the generated `Makefile.in'.
* tests/confh4.test: Relax the grepping of Makefile.in w.r.t.
white spaces.  Do not create usless dummy source file `foo.c'
and useless dummy header file `acconfig.h'.
(configure.in): Remove superfluous call to `AC_OUTPUT'.
* tests/confh6.test: Add trailing `:' command.
* tests/confh7.test: In comments, add reference to ...
* tests/confh8.test: ... this new test, "semantic" sister
of `confh7.test'.
* tests/Makefile.am (TESTS): Updated.
Prompted by a report from Ralf Wildenhues.