[RTG][python] fix populateDialectRTGTestSubmodule namespacing

[makslevental]

If CIRCT_INCLUDE_TESTS=OFF but this source file is compiled (only to eventually be DCEd) circt::python::populateDialectRTGTestSubmodule is malformed. With explicit namespace circt::python it works both ways.

[maerhart]

Given CIRCT_INCLUDE_TESTS=OFF, is there actually a situation in which this cpp file is compiled? Because there shouldn't. The CMakeLists only includes this file if tests are enabled.

Wouldn't compilation with CIRCT_INCLUDE_TESTS=ON still fail with this change because the RTGTest CAPI used in there also wasn't compiled?

We could enclose the function definition in an #ifdef CIRCT_INCLUDE_TESTS to make it robust against that. My thought was that not having it leads to a linker error pointing at a misconfigured build system (CMake) file.

[makslevental]

Given CIRCT_INCLUDE_TESTS=OFF, is there actually a situation in which this cpp file is compiled? Because there shouldn't. The CMakeLists only includes this file if tests are enabled.

admittedly I'm doing something unconventional - I'm building y'alls Python bindings but downstream of you. In such a setup, lib/Bindings/Python/RTGTestModule.cpp gets added to the CIRCTBindingsPythonExtension target (if CIRCT_INCLUDE_TESTS was on during build). Then when that same target is used in the downstream project it's compiled (even if CIRCT_INCLUDE_TESTS is off).

Wouldn't compilation with CIRCT_INCLUDE_TESTS=ON still fail with this change because the RTGTest CAPI used in there also wasn't compiled?

Nah because something like rtgtestCPUTypeGet is only exposed through your circt-c/Dialect/RTGTest.h header and then since populateDialectRTGTestSubmodule is never called (because that is already behind #ifdef CIRCT_INCLUDE_TESTS), the linker DCEs everything.

Anyway this is actually moot (I don't need to build all of CIRCT's dialects so I don't need this module at all actually). Up to you if you want to me to get this across the finish line or just close.

[teqdruid]

This change looks innocuous enough to me, though the discussion about whether or not it's compiled should be resolved into an issue or not.