dev: add fix test for mirror linter (#4598)

Author: alexandear

test/testdata/fix/in/mirror.go

@@ -0,0 +1,11 @@
+//golangcitest:args -Emirror
+//golangcitest:expected_exitcode 0
+package testdata
+
+import (
+	"unicode/utf8"
+)
+
+func foobar() {
+	_ = utf8.RuneCount([]byte("foobar"))
+}

test/testdata/fix/out/mirror.go

@@ -0,0 +1,11 @@
+//golangcitest:args -Emirror
+//golangcitest:expected_exitcode 0
+package testdata
+
+import (
+	"unicode/utf8"
+)
+
+func foobar() {
+	_ = utf8.RuneCountInString("foobar")
+}