Added WithReturnBackground tests and refactored tests in general

testExec and testMeanFirst now have WithReturnBackground versions.

Code has been refactored so that the algorithm executing is now in a separate method that sets the properties accordingly for that test.

A lot of TS_ASSERT_EQUALS tests have been chaged to TS_ASSERT_DELTA tests.

Refs #2057
Merge branch 'feature/2057_FlatBackground_return_test' into develop