tools/tests: Fix newly introduced Makefile

tools/tests: Fix newly introduced Makefile

Fix few issue with this new directory:
- clean generated files
- and ignore those generated files
- include the dependency files generated by `gcc`.
- rework prerequisites:
  "test-rangeset.o" also needs the generated files "list.h" and
  "rangeset.h". Technically, both only needs "harness.h" which needs
  the generated headers, but that's a bit simpler and the previous
  point will add the dependency on "harness.h" automatically.

This last point fix an issue where `make` might decide to build
"test-rangeset.o" before the other files are ready.

Fixes: 7bf777b42cad ("tootls/tests: introduce unit tests for rangesets")
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Signed-off-by: Anthony PERARD <anthony.perard@vates.tech>