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
Fixed GCC compiler warning

Fixed a compiler warning by removing an unused variable

Refs #2057
Moved some code and improved commenting

The method that runs the algorithm in different ways has been moved to the top.

A comment breifly mentioning what the test does has been added.

Refs #2057
Merge remote-tracking branch 'origin/feature/2057_FlatBackground_return_test'