dev: add test for grouper linter (#4585)

Author: alexandear

test/testdata/configs/grouper.yml

@@ -0,0 +1,3 @@
+linters-settings:
+  grouper:
+    import-require-grouping: true

test/testdata/grouper.go

@@ -0,0 +1,7 @@
+//golangcitest:args -Egrouper
+//golangcitest:config_path testdata/configs/grouper.yml
+package testdata
+
+import "fmt" // want "should only use grouped 'import' declarations"
+
+func dummy() { fmt.Println("dummy") }