Correct arithmetic test broadcasting CML directory names

Between #1076 and #1082 the broadcasting test for mathematical operators in analysis tests were all renamed from `TestBroadcast()` to `TestBroadcasting()`. This happened in all four test scripts under `lib/iris/test/unit/analysis/maths/`.

This has had the effect of changing the names of the results directories within which the CML for these four tests is saved, while keeping the out-of-date directories. This PR addresses this issue by renaming the four directories from `TestBroadcast` to `TestBroadcasting`.