run tests using the correct runtime for C# (#29302)

tools/run_tests/run_tests.py

@@ -907,11 +907,11 @@ class CSharpLanguage(object):
 
         specs = []
         for test_runtime in self.test_runtimes:
-            if self.args.compiler == 'coreclr':
+            if test_runtime == 'coreclr':
                 assembly_extension = '.dll'
                 assembly_subdir = 'bin/%s/netcoreapp3.1' % msbuild_config
                 runtime_cmd = ['dotnet', 'exec']
-            else:
+            elif test_runtime == 'mono':
                 assembly_extension = '.exe'
                 assembly_subdir = 'bin/%s/net45' % msbuild_config
                 if self.platform == 'windows':
@@ -921,6 +921,8 @@ class CSharpLanguage(object):
                     runtime_cmd = ['mono', '--arch=64']
                 else:
                     runtime_cmd = ['mono']
+            else:
+                raise Exception('Illegal runtime "%s" was specified.')
 
             for assembly in six.iterkeys(tests_by_assembly):
                 assembly_file = 'src/csharp/%s/%s/%s%s' % (