Add missing tests for String#codePointAt, concat and fromCharCodes (#1686)

Author: saulecabrera

tests/compiler/std/string.optimized.wat

@@ -13,6 +13,7 @@ assert("\xDFab" == "ßab");
 
 assert(str.length == 16);
 assert(str.charCodeAt(0) == 0x68);
+assert(str.codePointAt(1) == 105);
 assert(str.at(15) == str.charAt(15));
 assert(str.at(-1) == str.charAt(str.length - 1));
 assert(str.at(-str.length) == "h");
@@ -22,9 +23,13 @@ assert(!!"\0" == true);
 assert(!!"a" == true);
 
 assert(String.fromCharCode(0) == "\0");
+assert(String.fromCharCode(65600) == "@");
 assert(String.fromCharCode(54) == "6");
 assert(String.fromCharCode(0x10000 + 54) == "6");
 assert(String.fromCharCode(0xD800, 0xDF00) == "𐌀");
+assert(String.fromCharCodes([0, 54]) == "\06");
+assert(String.fromCharCodes([65, 66, 67]) == "ABC");
+assert(String.fromCharCodes([0xD834, 0xDF06, 0x61, 0xD834, 0xDF07]) == "\uD834\uDF06a\uD834\uDF07");
 
 assert(String.fromCodePoint(0) == "\0");
 assert(String.fromCodePoint(54) == "6");
@@ -770,6 +775,14 @@ assert(dtoa(1.1e+128) == "1.1e+128");
 assert(dtoa(1.1e-64) == "1.1e-64");
 assert(dtoa(0.000035689) == "0.000035689");
 
+
+// concat
+
+assert("Hello ".concat("World") == "Hello World");
+assert("".concat("bar") == "bar");
+assert("bar".concat("") == "bar");
+assert("".concat("") == "");
+
 // assert(dtoa(f32.MAX_VALUE) == "3.4028234663852886e+38"); // FIXME
 // assert(dtoa(f32.EPSILON) == "1.1920928955078125e-7"); // FIXME