Add the simplest test for submodules
Currently it fails with LFortran:
$ lfortran submodule_03.f90
Undefined symbols for architecture arm64:
"___module_submodule_03_mod_f", referenced from:
_main in a.out.tmp.o
ld: symbol(s) not found for architecture arm64
The submodule is effectively ignored, so the interface to f
does not
have an implementation.