test, lib: revised tests and shortened assert code

Changed the tests to check for correct throwing of
errors and combined if/else into one in assert.js
to make code concise.

lib/assert.js

@@ -394,9 +394,7 @@ function innerOk(fn, argLen, value, message) {
     } else if (message == null) {
       generatedMessage = true;
       message = getErrMessage(message, fn);
-    } else if (isNativeError(message)) {
-      throw message;
-    } else if (message instanceof Error) {
+    } else if (isNativeError(message) || message instanceof Error) {
       throw message;
     }
 

test/parallel/test-assert.js

@@ -55,51 +55,26 @@ assert.throws(() => a.ok(false), a.AssertionError, 'ok(false)');
   assert.ok(threw, 'Error: ok(false)');
 }
 
-// Throw message if the message is instanceof Error.
+// Thrown error should be the passed through error instance of the native error
 {
-  let threw = false;
   const context = vm.createContext();
-  try {
-    const errorConstructor = vm.runInContext('Error', context);
-    const customError = new errorConstructor('ok(false)');
-    assert.ok(false, customError.message);
-  } catch (e) {
-    threw = true;
-    assert.ok(e instanceof Error);
-  }
-  assert.ok(threw, 'Error: ok(false)');
-}
+  const error = vm.runInContext('new TypeError("custom error")', context);
 
-// Throw message if the message is instanceof TypeError.
-{
-  let threw = false;
-  const context = vm.createContext();
-  context.TypeError = TypeError;
-  try {
-   const TypeErrorConstructor = vm.runInContext('TypeError', context);
-   const customTypeError = new TypeErrorConstructor('ok(false)');
-   assert.ok(false, customTypeError);
-  } catch (e) {
-   threw = true;
-   assert.ok(e instanceof Error);
-  }
-  assert.ok(threw, 'TypeError: ok(false)');
+  assert.throws(() => assert(false, error), {
+    message: 'custom error',
+    name: 'TypeError'
+  });
 }
 
-// Throw message if the message is instanceof SyntaxError.
+// Thrown error should be the passed through error instance of the native error
 {
-  let threw = false;
   const context = vm.createContext();
-  context.SyntaxError = SyntaxError;
-  try {
-    const SyntaxErrorConstructor = vm.runInContext('SyntaxError', context);
-    const customSyntaxError = new SyntaxErrorConstructor('ok(false)');
-    assert.ok(false, customSyntaxError);
-  } catch (e) {
-    threw = true;
-    assert.ok(e instanceof Error);
-  }
-  assert.ok(threw, 'SyntaxError: ok(false)');
+  const error = vm.runInContext('new SyntaxError("custom error")', context);
+
+  assert.throws(() => assert(false, error), {
+    message: 'custom error',
+    name: 'SyntaxError'
+  });
 }
 
 a(true);