Don't rely on BOUT_PYTHONPATH, as not available always

BOUT_PYTHONPATH is not available to CI, so explicitly map both the build and source directories using global CMake variables.

Author: tomchapman

tests/run_integrated_tests.sh

@@ -8,4 +8,4 @@ export PYTHONPATH="@PROJECT_BINARY_DIR@/tools/pylib:@PROJECT_SOURCE_DIR@/tools/p
 cmake --build "$PROJECT_ROOT"
 
 pytest -m "not serial" --cache-clear -n auto --dist=loadgroup -q "$PROJECT_ROOT/tests/integrated" "$@"
-pytest -m serial "$PROJECT_ROOT/tests/integrated" "$@"
+pytest -m serial "$PROJECT_ROOT/tests/integrated" "$@"