add string concat test to basic.zig from misc.zig!

This means it is passing in stage2!

test/behavior/basic.zig

@@ -162,3 +162,7 @@ fn fA() []const u8 {
 fn fB() []const u8 {
     return "b";
 }
+
+test "string concatenation" {
+    try expect(mem.eql(u8, "OK" ++ " IT " ++ "WORKED", "OK IT WORKED"));
+}