Move test executable files produced by CMake to the base folder

Move test executable files produced by CMake to the base folder

Issue #281 (bdwgc).

This is done by moving the content of cord/CMakeLists.txt and
tests/CMakeLists.txt files to the base CMakeLists.txt.

* CMakeLists.txt [build_tests && build_cord]: Move content from
cord/CMakeLists.txt.
* CMakeLists.txt [build_tests && build_cord] (cordtest, de): Add
"cord/" to .c files in add_executable.
* CMakeLists.txt [build_tests]: Move content from tests/CMakeLists.txt.
* CMakeLists.txt [build_tests] (gctest, hugetest, leaktest, middletest,
realloc_test, smashtest, tracetest, test_cpp, disclaim_bench,
disclaim_test, disclaim_weakmap_test, disclaim_weakmap_test): Add
"tests/" to .c and .cc files in add_executable.
* Makefile.am (EXTRA_DIST): Remove cord/CMakeLists.txt,
tests/CMakeLists.txt.
* doc/README.cmake: Update info about CMakeLists.txt files (there is
just one file now).
* cord/CMakeLists.txt: Remove file.
* tests/CMakeLists.txt: Likewise.