build: no more recursion for 'tests' subdir

build: no more recursion for 'tests' subdir

* Makefile.am (SUBDIRS): Drop 'tests'.  Adjust comments.
(DISTCLEANFILES, MAINTAINERCLEANFILES): Define to empty, to be
updated later.
(MAINTAINERCLEANFILES): Adjust later definition to use '+='
rather than '='.
($(srcdir)/tests/local.mk): Include this.
* configure.ac (AC_CONFIG_FILES): Drop 'tests/Makefile'.
* lib/freeze.mk ($(AUTOM4TE_CFG)): Drop now-redundant remake rule.
($(build_libdir)/m4sugar/version.m4): Likewise.
* tests/Makefile.am: Rename ...
* tests/local.mk: ... like this, and adjust (quite heavily).
* tests/mktests.sh: Adjust to generate output files and temporary
files in the tests subdirectory rather than in the current
directory.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>