Add 'tests' subdir to gersemi defs

Author: ManifoldFR

Diff for: .gersemirc

@@ -1,4 +1,4 @@
-definitions: [./CMakeLists.txt, ./cmake]
+definitions: [./CMakeLists.txt, ./cmake, ./tests]
 line_length: 80
 indent: 2
 warn_about_unknown_commands: false