Automatically update generated files if UPDATE_SNAPSHOTS=1 (#243)

If you do `UPDATE_SNAPSHOTS=1 go test ./...` that will:
1. run the snapshot tests, updating any changed snapshots
2. run the integration tests and (if you have tokens) example tests
3. check that the code for the integration tests and example tests is
up-to-date

Step 3 is not strictly a snapshot test; the generated code is actually
checked in and used. But, I mean, it's basically the same! So now, we
also update it if you asked to update snapshots, which is hopefully a
little more convenient.

Fixes #212.

Test plan:
Make a trivial change to `example/generated.go`; `go test ./...` should
now fail. `UPDATE_SNAPSHOTS=1 go test ./...` should also fail, but say
it updated the snapshot, and the change should be reverted. Run `go test
./...` again; it should pass again.

docs/CONTRIBUTING.md

@@ -40,9 +40,7 @@ Notes for contributors:
 - If `GITHUB_TOKEN` is available in the environment, it also checks that the example returns the expected output when run against the real API.  This is configured automatically in GitHub Actions, but you can also use a [personal access token](https://docs.github.com/en/github/authenticating-to-github/creating-a-personal-access-token) with no scopes.  There's no need for this to cover anything in particular; it's just to make sure the example in fact works.
 - Tests should use `testify/assert` and `testify/require` where convenient (when making many simple assertions).
 
-If you update any code-generation logic or templates, even if no new tests are needed you'll likely need to:
-- Run `UPDATE_SNAPSHOTS=1 go test ./...` to update the [cupaloy](https://github.com/bradleyjkemp/cupaloy) snapshots.
-- Run `go generate ./...` to update the genqlient-generated files used in integration tests and documentation.
+If you update any code-generation logic or templates, even if no new tests are needed you'll likely need to run `UPDATE_SNAPSHOTS=1 go test ./...` to update the [cupaloy](https://github.com/bradleyjkemp/cupaloy) snapshots and the genqlient-generated files used in integration tests and documentation.
 
 ## Finding your way around
 

internal/integration/util.go

@@ -51,6 +51,14 @@ func RunGenerateTest(t *testing.T, relConfigFilename string) {
 			if testing.Verbose() {
 				t.Errorf("got:\n%s\nwant:\n%s\n", content, expectedContent)
 			}
+			if os.Getenv("UPDATE_SNAPSHOTS") == "1" {
+				err = os.WriteFile(filename, content, 0o644)
+				if err != nil {
+					t.Errorf("unable to update generated file %s: %v", filename, err)
+				} else {
+					t.Errorf("updated generated file for %s", filename)
+				}
+			}
 		}
 	}
 }