test: add file expected by example

Go 1.24 seems to expect that go tests support for examples should be
named to match an actual identifier, so add that identifier so that the
test for :GoTest for examles will work.

Author: bhcleek

autoload/go/test-fixtures/test/src/example/example.go

@@ -0,0 +1,5 @@
+package main
+
+func HelloWorld() string {
+	return "not the hello you're looking for"
+}

autoload/go/test-fixtures/test/src/example/go.mod

@@ -1,3 +1,3 @@
 module vim-go.test/example
 
-go 1.16
+go 1.24