Rollup merge of #122644 - Nadrieril:complexity-tests, r=compiler-errors

pattern analysis: add a custom test harness

There are two features of the pattern analysis code that are hard to test: the newly-added pattern complexity limit, and the computation of arm intersections. This PR adds some crate-specific tests for that, including an unmaintainable but pretty macro to help construct patterns.

r? `````@compiler-errors`````

Author: matthiaskrgr