docs: note about integration test in readme, helpful error message

runner/cf-driver-go/README.md

@@ -40,12 +40,23 @@ go get github.com/google/some-mod/pkg
 
 ## Running tests
 
+### Unit tests
+
 The simplest way to run tests—or the one with the least typing, at least—is with `make`.
 
 ```sh
 make test
 ```
 
+### Integration tests
+
+We only have one integration test right now, and to get it running you'll need to do a bit of local setup.
+
+1. You will need to first get a username & password for some space on cloud.gov that has at least one app.
+1. Then you can add those credentials to `./cg/testdata/.cg_creds` in the style of the `.cg_creds.sample` file there.
+1. Run the test with `make integration`, which should give you an error and, in its output, show you what the resulting JSON looks like.
+1. Copy that JSON result over to the last line of your `.cg_creds` file and run `make integration` again, this time it should succeed.
+
 ## Builds
 
 You can also run a build with `make` (`make build`, or simply `make`), but it won't do you much good because we aren't set up to do anything with an executable yet.

runner/cf-driver-go/cg/cg_integration_test.go

@@ -16,9 +16,14 @@ func Test_CFAdapter_GetApps(t *testing.T) {
 	var u, p, want string
 	var l int
 
-	f, err := os.Open("./testdata/.cg_creds")
+	path := "./testdata/.cg_creds"
+	f, err := os.Open(path)
 	if err != nil {
-		t.Errorf("Error opening testdata file = %v", err)
+		t.Errorf(
+			"Error opening testdata file = %v\n\033[1;33mDid you forget to create `%v`?\033[0m",
+			err, path,
+		)
+		return
 	}
 	defer f.Close()