Test for source_map memory leaks

[ef4]

I am reasonably confident that the span interning system in swc is going to hold onto an ever-growing amount of data if you continuously do rebuilds using the same Preprocess instance.

But every example in swc itself that I can find also does things that way. swc holds the source maps in a static lifetime and calls new_source_file again and again on it, which is equivalent to what we're doing.

It would be good for someone to exercise this on a bigger build and watch the memory consumption as soon as we have it wired into a useful system.