Added permutations and ranges to test defines

This is really more work for the bench runner. With this change defines
can be manipulated at a rather high level at runtime. Which should be
useful for generating benchmarks across various dimensions.

The define grammar in the test_runner is now a bit more powerful,
accepting:

1. A single value: -DN=42
2. A list of values, which get permuted: -DN=1,2,3
3. A range: -DN=range(10)
4. Some combo: -DN=1,2,range(3,0,-1)

This is more complex in the test .toml defines, which can also be C
expressions:

1. A single value: define=42
2. A single expression: define='42*42'
3. A list: define=[1,2,3]
4. A comma separated string: define='1,2,3'
5. A range: define='42*range(10)'
6. This mess: define=[1,2,'3,4,range(2)*range(2)+3']

runners/test_runner.h

@@ -43,7 +43,7 @@ struct test_case {
     test_flags_t flags;
     size_t permutations;
 
-    const test_define_t *const *defines;
+    const test_define_t *defines;
 
     bool (*filter)(void);
     void (*run)(struct lfs_config *cfg);