[lua] Add regression test for pcall return wrapping

[jdonaldson]

Summary

• Follow-up to [lua] Remove unnecessary do...end wrappers around tail returns #12601
• Adds a regression test (Issue7350) for the in_pcall guard that ensures do return end wrapping is preserved inside try-catch blocks (pcall callbacks), where the compiler appends return _hx_pcall_default after user code.
• Also tests that tail returns use bare return without wrapping.

Test plan

• make haxe builds successfully
• All 11,711 Lua unit tests pass

[tobil4sk]

For this function, haxe currently generates:

___Main_Main_Fields_.returnInTryCatch = function(x) 
  local _hx_status, _hx_result = pcall(function() 
  
      if (x > 0) then 
        do return x * 2 end;
      end;
    return _hx_pcall_default
  end)
  if not _hx_status then 
    local _g = _hx_result;
    return -1;
  elseif _hx_result ~= _hx_pcall_default then
    return _hx_result
  end;
  return 0
end

from what I can tell in this case the do end doesn't make much of a difference?

[tobil4sk]

This test doesn't really do much, because even if there is a do ... end it's still going to pass. It's most useful to test if there is a case where the compiler missing out do ... end would result in broken output.

[jdonaldson]

Good point — rewrote the test to exercise actual pcall return propagation edge cases instead. Now covers returning null/false/zero through pcall (must not be confused with _hx_pcall_default sentinel), fall-through where try body has no return (code after try-catch must execute), nested try-catch, try-catch in loops, and exception handling paths.