tests: use absolute path of source files

Replace ../$srcdir with $abs_srcdir in gettext-tools tests
to add support for configuration with absolute path.
tests: use absolute path of source files

Replace ../$top_srcdir with $abs_top_srcdir in gettext-tools tests
to add support for configuration with absolute path.
Update ChangeLog entry.
tests: fix "make check" under VPATH builds

Previously, we generated init.cfg in $builddir, while tests/init.sh
reads its configuration file (init.cfg) from $srcdir.  This patch
moves the auto-generated part into a separate file
($builddir/init-env) and read the file from $srcdir/init.cfg.