Improve and extend tests `suffix*.test'.

Improve and extend tests `suffix*.test'.

* tests/suffix.test: Check that suffix rules for C compilation are
only included once.  Try also with a static library.
* tests/suffix2.test: Add a new grep to help potential debugging.
Do not run automake with the `--add-missing' options, since we
already create all the needed auxiliary files.  Try also *without*
the `no-dependencies' automake option.
* tests/suffix4.test: Make grepping of Makefile.in stricter.
* tests/suffix3.test: Rewritten to run also autoconf, ./configure
and make.
* tests/suffix5.test: Likewise.
* tests/suffix6.test: Fix botched recipe indentation (eight spaces
were used instead of a tabulation character).  Extend to check
that `.obj' is handled like `.$(OBJEXT)' (as is done for `.o').
Improved parsing & grepping of generated Makefile.in.  Other minor
fixes and improvements.
* tests/suffix10.test: Move some checks in Makefile.am.  Also run
"make all".
* tests/suffix12.test: Likewise, and account for VPATH issues in
weaker make implementations.
* tests/suffix11.test: Likewise.  Also, run "make distcheck", for
completeness, and related changes.
* tests/suffix8.test: Likewise.  Also, do not put `gcc' anymore
in $required.
* tests/suffix13.test: Do not use the `--force-missing' automake
option unnecessarily.
* tests/suffix6b.test: New test, semantic sister of `suffix6.test'.
* tests/suffix6c.test: Likewise.
* tests/Makefile.am (TESTS): Updated.