ADD_PYTHON_UNIT_TEST: allow multiple paths in argn (#240)

Multiple paths can be set in PYTHONPATH, but they have to be joined by
`os.pathsep`(":" on unix, ";" on windows)
https://docs.python.org/3/using/cmdline.html#envvar-PYTHONPATH

This commit fix the concatenation with any pre-existing environment
variables, and allows to set several folders (relative to the project
build tree) inside this PYTHONPATH.

eg. if you build a module in src/ and a test module in tests/, you can
import them both in a test with:
ADD_PYTHON_UNIT_TEST("test-python" "tests/main.py" "src" "tests")

test.cmake

@@ -86,10 +86,10 @@ MACRO(ADD_UNIT_TEST NAME SOURCE)
 ENDMACRO(ADD_UNIT_TEST NAME SOURCE)
 
 #.rst:
-# .. command:: ADD_PYTHON_UNIT_TEST (NAME SOURCE [MODULE_PATH])
+# .. command:: ADD_PYTHON_UNIT_TEST (NAME SOURCE [MODULES...])
 #
 #   Add a test called `NAME` that runs an equivalent of ``python ${SOURCE}``,
-#   optionnaly with a `PYTHONPATH` set to `CMAKE_BINARY_DIR/MODULE_PATH` if `MODULE_PATH` is set.
+#   optionnaly with a `PYTHONPATH` set to `CMAKE_BINARY_DIR/MODULE_PATH` for each MODULES
 #   `SOURCE` is relative to `PROJECT_SOURCE_DIR`
 #
 #   .. note:: :command:`FINDPYTHON` should have been called first.
@@ -98,16 +98,23 @@ MACRO(ADD_PYTHON_UNIT_TEST NAME SOURCE)
   ADD_TEST(NAME ${NAME} COMMAND ${PYTHON_EXECUTABLE} "${PROJECT_SOURCE_DIR}/${SOURCE}")
   SET(PYTHONPATH)
 
-  SET(MODULE_PATH "${ARGN}")  # ARGN is not a variable
-  IF(MODULE_PATH)
+  SET(MODULES "${ARGN}")  # ARGN is not a variable
+  FOREACH(MODULE_PATH IN LISTS MODULES)
     LIST(APPEND PYTHONPATH "${CMAKE_BINARY_DIR}/${MODULE_PATH}")
-  ENDIF(MODULE_PATH)
-  
+  ENDFOREACH(MODULE_PATH IN LISTS MODULES)
+
   IF(DEFINED ENV{PYTHONPATH})
-    SET(PYTHONPATH "${PYTHONPATH}:$ENV{PYTHONPATH}")
+    LIST(APPEND PYTHONPATH "$ENV{PYTHONPATH}")
   ENDIF(DEFINED ENV{PYTHONPATH})
 
-  SET(ENV_VARIABLES "PYTHONPATH=${PYTHONPATH}")
+  # get path separator to join those paths
+  EXECUTE_PROCESS(COMMAND
+      "${PYTHON_EXECUTABLE}" "-c" "import os; print(os.pathsep)"
+      OUTPUT_VARIABLE PATHSEP
+      OUTPUT_STRIP_TRAILING_WHITESPACE)
+
+  STRING(REPLACE ";" ${PATHSEP} PYTHONPATH_STR "${PYTHONPATH}")
+  SET(ENV_VARIABLES "PYTHONPATH=${PYTHONPATH_STR}")
   IF(APPLE)
     LIST(APPEND ENV_VARIABLES "LD_LIBRARY_PATH=$ENV{LD_LIBRARY_PATH}")
     LIST(APPEND ENV_VARIABLES "DYLD_LIBRARY_PATH=$ENV{DYLD_LIBRARY_PATH}")