Start refactoring client_test.py to use unittest module

In preparation for splitting the test into smaller pieces,
make the current test a single case in a unittest.TestCase
fixture.
Speed up client_test.py by reducing the number of PBKDF2 iterations

Add -low-security flag to the client which reduces the number
of PBKDF2 iterations to 10.
Remove unused checkErr() func
Start splitting client_test.py test fixture into separate test cases

Also use a different vault directory for each test case
to avoid dependencies between cases.
Add tests for folder manipulation and renaming items