cbrt* tests: More tests.

cbrt* tests: More tests.

* tests/test-cbrt.h: New file.
* tests/test-cbrt.c: Include <float.h> and test-cbrt.h.
(main): Invoke test_function.
* tests/test-cbrtf.c: Include <float.h> and test-cbrt.h.
(main): Invoke test_function.
* tests/test-cbrtl.c: Include <float.h> and test-cbrt.h.
(main): Invoke test_function.
* modules/cbrt-tests (Files): Add tests/test-cbrt.h, tests/randomd.c.
(Makefile.am): Add randomd.c to test_cbrt_SOURCES.
* modules/cbrtf-tests (Files): Add tests/test-cbrt.h, tests/randomf.c.
(Makefile.am): Add randomf.c to test_cbrtf_SOURCES.
* modules/cbrtl-tests (Files): Add tests/test-cbrt.h, tests/randoml.c.
(Depends-on): Add 'float'.
(Makefile.am): Add randoml.c to test_cbrtl_SOURCES.