Improve tests stability for DownloadInstrumentTest

[NickDraper]

test_execOrphanedFile: files not cleaned up

20:16:27 491/1746 Test #719: DataHandlingTest_DownloadInstrumentTest ......................***Failed 4.55 sec 20:16:27 FrameworkManager-[Notice] Welcome to Mantid 3.9.20170215.1925 20:16:27 FrameworkManager-[Notice] Please cite: http://dx.doi.org/10.1016/j.nima.2014.07.029 and this release: http://dx.doi.org/10.5286/Software/Mantid 20:16:27 Running 4 tests.DownloadInstrument-[Notice] DownloadInstrument started 20:16:27 DownloadInstrument-[Notice] Downloading 2 files from the instrument repository 20:16:27 DownloadInstrument-[Notice] DownloadInstrument successful, Duration 1.75 seconds 20:16:27 ..DownloadInstrument-[Notice] DownloadInstrument started 20:16:27 DownloadInstrument-[Error] Error in execution of algorithm DownloadInstrument 20:16:27 DownloadInstrument-[Error] Unable to parse server JSON file "C:\Users\builder\AppData\Roaming\mantidproject\mantid\instrument\github.json" 20:16:27 20:16:27 In DownloadInstrumentTest::test_execOrphanedFile: 20:16:27 C:/Jenkins/workspace/pull_requests-win7/Framework/DataHandling/test/DownloadInstrumentTest.h:160: Error: Assertion failed: alg.isExecuted() 20:16:27 C:/Jenkins/workspace/pull_requests-win7/Framework/DataHandling/test/DownloadInstrumentTest.h:143: Error: Test failed: The expected number of files downloaded was wrong. 20:16:27 C:/Jenkins/workspace/pull_requests-win7/Framework/DataHandling/test/DownloadInstrumentTest.h:143: Error: Expected (runDownloadInstrument() == 2), found (0 != 2) 20:16:27 Failed 1 of 4 tests 20:16:27 Success rate: 75%

[NickDraper]

One test was not cleaning up after itself, and another was incomplete and pointless and will be removed

[NickDraper]

As an aside the last commit makes the test run over 20x faster