e2e test harness improvements for snapshot tests

[xunilrj]

Now that we have snapshot tests, we can have more complete tests if the test harness emits some "internals" like:

1. focused IR;

we can emit IR very easily already with --print-ir, but we print the IR from the whole application when sometimes we only want to check the IR from a specific function.

2. diff for an optimization pass and the previous pass;

Would be much easier to test if an optimization pass is doing what we want if we could print a simple diff from one pass to the next one.

3. constant, configurable and "dynamic section" sizes;

This would be useful for testing dynamic configurable, but also to guarantee that variable-sized encoding (for enums, for example) always pad its buffer when saving direct configurables.