Rework setting of LD_LIBRARY_PATH for tests.

* testsuite/Makefile.in (check): Pass only TEST_SHLIB_DIR
to the run-tests script, and leave setting of LD_LIBRARY_PATH and
related variables to that script.
* examples/Makefile.in (check): Likewise.

* run-tests: Check TEST_SHLIB_DIR, and set up LD_LIBRARY_PATH and
related member variables.

* config.make.in (abs_top_builddir, TEST_SHLIB_DIR): New variables.