Fix IOException when running code coverage.

Running unit tests with code coverage would intermittently throw an
IOException:

System.IO.IOException: The process cannot access the file
'...OpenCover\coverage.xml'

src/AddIns/Analysis/CodeCoverage/Project/Src/CodeCoverageResultsReader.cs

@@ -52,8 +52,9 @@ namespace ICSharpCode.CodeCoverage
 		
 		CodeCoverageResults ReadCodeCoverageResults(string fileName)
 		{
-			TextReader reader = fileSystem.OpenText(FileName.Create(fileName));
-			return new CodeCoverageResults(reader);
+			using (TextReader reader = fileSystem.OpenText(FileName.Create(fileName))) {
+				return new CodeCoverageResults(reader);
+			}
 		}
 		
 		public IEnumerable<string> GetMissingResultsFiles()

src/AddIns/Analysis/CodeCoverage/Project/Src/RunTestWithCodeCoverageCommand.cs

@@ -119,7 +119,7 @@ namespace ICSharpCode.CodeCoverage
 		
 		void DisplayCodeCoverageResults(CodeCoverageResultsReader coverageResultsReader)
 		{
-			foreach (CodeCoverageResults result in coverageResultsReader.GetResults()) {
+			foreach (CodeCoverageResults result in GetResults(coverageResultsReader)) {
 				DisplayCodeCoverageResults(result);
 			}
 			foreach (string missingFile in coverageResultsReader.GetMissingResultsFiles()) {
@@ -127,6 +127,11 @@ namespace ICSharpCode.CodeCoverage
 			}
 		}
 
+		IEnumerable<CodeCoverageResults> GetResults(CodeCoverageResultsReader coverageResultsReader)
+		{
+			return SD.MainThread.InvokeIfRequired(() => coverageResultsReader.GetResults().ToList());
+		}
+		
 		void DisplayCodeCoverageResults(CodeCoverageResults results)
 		{
 			SD.MainThread.InvokeIfRequired(() => CodeCoverageService.ShowResults(results));