Remove ES_NEXT macro (#4915)

- remove all '#JERRY_ESNEXT' macro
- remove 5.1 build profile, update test runner accordingly (Note: all builtins are turn on by default)
- move tests from tests/jerry/esnext into tests/jerry, concatenate files with same names
- add skiplist to some snapshot tests that were supported only in 5.1
- fix doxygen issues that were hidden before (bc. of es.next macro)

Co-authored-by: Martin Negyokru negyokru@inf.u-szeged.hu
JerryScript-DCO-1.0-Signed-off-by: Adam Szilagyi aszilagy@inf.u-szeged.hu

tests/jerry/string-prototype-at.js

@@ -0,0 +1,50 @@
+/* Copyright JS Foundation and other contributors, http://js.foundation
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+var str = 'This is an Apple';
+
+/* Basic tests */
+var index = str.at(0);
+assert(index === 'T');
+assert(str[index] === undefined);
+
+assert(str.at(str.length) === undefined);
+assert(str.at(str.length+1) === undefined);
+assert(str.at(str.length-1) === 'e');
+assert(str.at("1") === 'h');
+assert(str.at(-1) === 'e');
+assert(str.at("-1") === 'e');
+assert(str.at("-20") === undefined);
+
+try {
+  String.prototype.at.call(undefined)
+  assert (false);
+} catch(e) {
+  assert(e instanceof TypeError);
+}
+
+var obj = {toString: function() { return "Apple"; } };
+obj.at = String.prototype.at;
+assert(obj.at(0) === 'A');
+
+/* BigInteger as num and char */
+assert(str.at("1n") === 'T')
+
+try {
+  str.at (10n);
+  assert (false);
+} catch (e) {
+  assert (e instanceof TypeError);
+}