Issue with clang format in the tests

[jmcarcell]

#358 introduced a sort of hidden dependency on clang-format as now there are tests which will generate a model with clang format and the -fallback-style=llvm argument is not supported by older versions of clang-format. This breaks the nightly build for key4hep, because llvm is not a dependency. I think we can agree that the tests should run without llvm since it's not a dependency of podio. These are some solutions I can think about:

• In the CMake step, when generating the original model save some variable to be checked later at the tests to make sure that we run the tests with the same arguments as now
• Alternatively we could just parse the dumped model and the original one even in a python one liner and compare the results. It's a slightly weaker test but there are tests already for the generation from the model

[tmadlener]

This could also be fixed by making the check for discovering clang-format more robust. Currently, it just checks for the presence, but does not try to actually run something:

podio/python/podio_class_generator.py

Lines 40 to 49 in dc9b6ba

	def get_clang_format():
	"""Check if clang format is available and if so get the list of arguments to
	invoke it via subprocess.Popen"""
	try:
	cformat_exe = subprocess.check_output(['which', 'clang-format']).strip()
	return [cformat_exe, "-style=file", "-fallback-style=llvm"]
	except subprocess.CalledProcessError:
	print("ERROR: Cannot find clang-format executable")
	print(" Please make sure it is in the PATH.")
	return []

Actually trying to run clang-format with all the flags we run it later should already fix all issues, because in case that does not succeed the generation will simply not format generated code.

We actually do something like that in the cmake macros:

podio/cmake/podioMacros.cmake

Line 154 in dc9b6ba

execute_process(COMMAND ${CLANG_FORMAT_EXE} --fallback-style OUTPUT_VARIABLE CLANG_FALLBACK_STYLE ERROR_VARIABLE CLANG_FALLBACK_STYLE_ERR)

Overall, even that doesn't cover all possibilities yet, because the contents of .clang-format are in principle LLVM version dependent, and we require probably at least 9 or 10.