Remove use of slurp in load-props.

Fix test cases.
Add read-config-str fn that reads from a Properties string.

Add related test-cases.
Minor cleanup to remove duplication in argument handling in read-config-str, and test for write-config
Removed the mock function and now we test only the read-config-str function in all cases except one where we test with a real file.
Reverted a previous change - seeing arguments-info in the docs is a better idea!