CMake: if pylint is available, run it as part of the test-suite

This introduces a stub-implementation (fake) that mimics the
API offered by libcalamares (the library is actually exposed
to Python via Boost::Python, so it doesn't act like a C-extension).
Using that stub-implementation, we can check Python modules for
validity as part of the test-suite.

The stub-implementation is needed, because otherwise every
Python module already fails at `import libcalamares`.

- stub-implement the API that is actually used by the Python modules
- in globalstorage, be slightly smart about what keys are being
  requested (so that e.g. all the modules that handle partitions
  information get an empty list and can manipulate that, instead of
  erroring out when they get a string)

CMakeModules/CalamaresAddModuleSubdirectory.cmake

@@ -42,6 +42,18 @@ include( CalamaresCheckModuleSelection )
 
 set( MODULE_DATA_DESTINATION share/calamares/modules )
 
+# We look for Pylint (just once) so that unittests can be added that
+# check the syntax / variables of Python modules. This should help
+# avoid more typo's-in-releases.
+if(BUILD_TESTING AND NOT PYLINT_COMMAND_SEARCHED)
+    set(PYLINT_COMMAND_SEARCHED TRUE)
+    find_program(
+        PYLINT_COMMAND
+        NAMES pylint3 pylint
+        PATHS $ENV{HOME}/.local/bin
+        )
+endif()
+
 function( _calamares_add_module_subdirectory_impl )
     set( SUBDIRECTORY ${ARGV0} )
 
@@ -241,6 +253,19 @@ function( _calamares_add_module_subdirectory_impl )
         if ( EXISTS ${_testdir}/CMakeTests.txt AND NOT EXISTS ${_mod_dir}/CMakeLists.txt )
             include( ${_testdir}/CMakeTests.txt )
         endif()
+        if ( PYLINT_COMMAND AND MODULE_INTERFACE MATCHES "python" )
+            # Python modules get an additional test via pylint; this
+            # needs to run at top-level because the ci/libcalamares directory
+            # contains API stubs.
+            #
+            # TODO: the entry point is assumed to be `main.py`, but that is
+            #       configurable through module.desc
+            add_test(
+                NAME lint-${SUBDIRECTORY}
+                COMMAND env PYTHONPATH=ci: ${PYLINT_COMMAND} -E ${_mod_dir}/main.py
+                WORKING_DIRECTORY ${CMAKE_SOURCE_DIR}
+                )
+        endif()
     endif()
 endfunction()