Reintroduced test-defines into the new test_runner

This moves defines entirely into the runtime of the test_runner,
simplifying thing and reducing the amount of generated code that needs
to be build, at the cost of limiting test-defines to uintmax_t types.

This is implemented using a set of index-based scopes (created by
test.py) that allow different layers to override defines from other
layers, accessible through the global `test_define` function.

layers:
1. command-line overrides
2. per-case defines
3. per-geometry defines

.gitignore

@@ -4,6 +4,8 @@
 *.a
 *.ci
 *.csv
+*.t.c
+*.a.c
 
 # Testing things
 blocks/