[EH][test] Slice test code into different functions (WebAssembly#6177)

We ported basic tests to `test/lit/basic/` in WebAssembly#6160, but comparing
`CHECK` lines with the test code for long functions is not easy, even
though it wouldn't necessarily be worse than the the separate files we
used to have in `test/`. This slices `exception-handling.wast` into
functions so that the `CHECK` lines are easy to check.