Simplify utility tests for flow

[kidrahahjo]

Feature description

We test a lot of utility behaviour in single functions in https://github.com/glotzerlab/signac-flow/blob/master/tests/test_util.py.

Related to the work discussed in #611

Proposed solution

Break down the tests for _bidict and Template Filters into smaller tests.

Additional context

test_calc_memory tests the result of calc_memory when operations are run in series and in parallel in a single function.
Write two seperate functions test_calc_memory_serial and test_calc_memory_parallel and test the results accordingly.