wasm2c: custom trap handler function needs testing

Per discussion in #2194

@sbc100 @keith We can definitely write a custom test for this, but would you have thoughts on the best way to add this to the existing harness. The test probably has to look something like this.

**Wasm module**
memory size = 1
func foo that accesses memory index 2

**Compilation**
Compile above module with
- flag `-DWASM_RT_TRAP_HANDLER=trap_handler`
- A main func that invokes the foo
- A `trap_handler` func

Test succeeds if trap_handler is called and fails otherwise.

Start by locating the existing wasm2c test harness and the path that compiles modules with a custom trap handler. Add the described module, main function, trap_handler, and compilation flag, then verify the test succeeds only when trap_handler is called.