Adding unit tests some of the Analysers

ConcatenationAllocation, EnumeratorAllocation & ExplicitAllocation all
have unit tests, covering all the scenarios in the Analysers
Covering more scenarios and some bug fixes
Include more null checks

as per comments on the pull request https://github.com/mattwarren/RoslynClrHeapAllocationAnalyzer/commit/0f9f62fda6fee5c8ccbde5b644fb1c86d599f24d#commitcomment-8233597 and https://github.com/mattwarren/RoslynClrHeapAllocationAnalyzer/commit/0f9f62fda6fee5c8ccbde5b644fb1c86d599f24d#commitcomment-8233588
Merge pull request #3 from mattwarren/master

Adding unit test infrastructure and some actual tests