Simplify some region analysis tests

[gafter]

In a recent code review @jcouv suggested, seeing reams of code like this:

            Assert.Equal("x", GetSymbolNamesJoined(dataFlowAnalysisResults.VariablesDeclared));
            Assert.Equal(null, GetSymbolNamesJoined(dataFlowAnalysisResults.AlwaysAssigned));
            Assert.Equal(null, GetSymbolNamesJoined(dataFlowAnalysisResults.Captured));
            Assert.Equal(null, GetSymbolNamesJoined(dataFlowAnalysisResults.CapturedInside));
            Assert.Equal(null, GetSymbolNamesJoined(dataFlowAnalysisResults.CapturedOutside));

Not related to this PR: consider adding a helper method in this test file to do the assert plus the GetSymbolNamesJoined, so that GetSymbolNamesJoined doesn't have to be repeated.

It could look like: ValidateSymbolNames("x", dataFlowAnalysisResults.VariablesDeclared)