doit: simplify tests by using t.Setenv (#1422)

doit_test.go

@@ -132,34 +132,22 @@ func TestCommandName(t *testing.T) {
 	t.Run("snap name set to goland", func(t *testing.T) {
 		const snapName = "goland"
 
-		if err := os.Setenv("SNAP_NAME", snapName); err != nil {
-			t.Errorf("failed to set environment variable: %#v", err)
-		}
+		t.Setenv("SNAP_NAME", snapName)
 
 		// When run under `go test`, os.Args[0] will be different every time,
 		// so only check that the Snap name is not "goland".
 		if actual := CommandName(); actual == snapName {
 			t.Errorf("expected name not to equal %s, got %s", snapName, actual)
 		}
-
-		if err := os.Unsetenv("SNAP_NAME"); err != nil {
-			t.Errorf("failed to unset environment variable: %#v", err)
-		}
 	})
 
 	t.Run("snap name set to doctl", func(t *testing.T) {
 		const expected = "doctl"
 
-		if err := os.Setenv("SNAP_NAME", "doctl"); err != nil {
-			t.Errorf("failed to set environment variable: %#v", err)
-		}
+		t.Setenv("SNAP_NAME", expected)
 
 		if actual := CommandName(); actual != expected {
 			t.Errorf("got %s, want %s", actual, expected)
 		}
-
-		if err := os.Unsetenv("SNAP_NAME"); err != nil {
-			t.Errorf("failed to unset environment variable: %#v", err)
-		}
 	})
 }