Reduce macro contents in `expect_pending_htlcs_forwardable*` macros

Reduce macro contents in `expect_pending_htlcs_forwardable*` macros

The `expect_pending_htlcs_forwardable*` macros don't need to be
macros so here we move much of the logic in them to a function and
leave the macro in place to avoid touching every line of code in
the tests.

This reduces the `--profile=test --lib` `Zpretty=expanded` code
size from 301,915 LoC to 295,294 LoC.