Copy test file to temporary directory before performing tests.
UnitTestReadWriteMeshJSON imports a collection from a JSON file, modifies it, and then writes the changes back to the imported file. This commit changes the testing behavior to first copy the file to a temporary location, then perform the test on the temporary file, and then remove the file. With this update, the testing data directory remains read-only.
!239 (merged) is a better fix for this issue.