compiletest: Automatically compare output by subset with runners

This commit updates compiletest to automatically compare test output
with subsets if a `--runner` argument is configured. Runners might
inject extra information on failures, for example a WebAssembly runtime
printing a wasm stack trace, which won't be in the output of a native
runtime. The output with a `--runner` argument, however, should still
have all the native output present.