Add tests for string helpers too, update timestamp strings tests

Fix string helpers calls for some edge cases

ReadMe.md

@@ -55,6 +55,18 @@ uv run --with corelibs --index egra-gitea=https://git.egplusww.jp/api/packages/P
 
 All python tests are the tests/ folder. They are structured by the source folder layout
 
+run them with
+
+```sh
+uv run pytest
+```
+
+Get a coverate report
+
+```sh
+uv run pytest --cov=corelibs
+```
+
 ### Other tests
 
 In the test-run folder usage and run tests are located