internaltests: Fix tests for /utf-8 removal

Now that we don't insert /utf-8 into the always args for MSVC < 19.00
we need to use a version > 19.00 for testing. This also means that
/Zc:__cplusplus will be added to the always args.

unittests/internaltests.py

@@ -224,22 +224,23 @@ class InternalTests(unittest.TestCase):
 
     def test_compiler_args_class_visualstudio(self):
         linker = mesonbuild.linkers.MSVCDynamicLinker(MachineChoice.HOST, [])
-        cc = VisualStudioCPPCompiler([], [], 'fake', MachineChoice.HOST, False, mock.Mock(), 'x64', linker=linker)
+        # Version just needs to be > 19.0.0
+        cc = VisualStudioCPPCompiler([], [], '20.00', MachineChoice.HOST, False, mock.Mock(), 'x64', linker=linker)
 
         a = cc.compiler_args(cc.get_always_args())
-        self.assertEqual(a.to_native(copy=True), ['/nologo', '/showIncludes', '/utf-8'])
+        self.assertEqual(a.to_native(copy=True), ['/nologo', '/showIncludes', '/utf-8', '/Zc:__cplusplus'])
 
         # Ensure /source-charset: removes /utf-8
         a.append('/source-charset:utf-8')
-        self.assertEqual(a.to_native(copy=True), ['/nologo', '/showIncludes', '/source-charset:utf-8'])
+        self.assertEqual(a.to_native(copy=True), ['/nologo', '/showIncludes', '/Zc:__cplusplus', '/source-charset:utf-8'])
 
         # Ensure /execution-charset: removes /utf-8
         a = cc.compiler_args(cc.get_always_args() + ['/execution-charset:utf-8'])
-        self.assertEqual(a.to_native(copy=True), ['/nologo', '/showIncludes', '/execution-charset:utf-8'])
+        self.assertEqual(a.to_native(copy=True), ['/nologo', '/showIncludes', '/Zc:__cplusplus', '/execution-charset:utf-8'])
 
         # Ensure /validate-charset- removes /utf-8
         a = cc.compiler_args(cc.get_always_args() + ['/validate-charset-'])
-        self.assertEqual(a.to_native(copy=True), ['/nologo', '/showIncludes', '/validate-charset-'])
+        self.assertEqual(a.to_native(copy=True), ['/nologo', '/showIncludes', '/Zc:__cplusplus', '/validate-charset-'])
 
 
     def test_compiler_args_class_gnuld(self):