Add CI check for test fixture diffs

[damemi]

#452 introduced a new diff when recording new fixtures:

$ exporter/collector/integrationtest$ go run cmd/recordfixtures/main.go
...
$ git diff
diff --git a/exporter/collector/integrationtest/testdata/fixtures/metrics_multi_project_expected.json b/exporter/collector/integrationtest/testdata/fixtures/metrics_multi_project_expected.json
index c513b6e..92ab158 100644
--- a/exporter/collector/integrationtest/testdata/fixtures/metrics_multi_project_expected.json
+++ b/exporter/collector/integrationtest/testdata/fixtures/metrics_multi_project_expected.json
@@ -1,7 +1,7 @@
 {
   "createTimeSeriesRequests": [
     {
-      "name": "projects/fake-other-project",
+      "name": "projects/fakeprojectid",
       "timeSeries": [
         {
           "metric": {
@@ -352,7 +352,7 @@
       ]
     },
     {
-      "name": "projects/fakeprojectid",
+      "name": "projects/fake-other-project",
       "timeSeries": [
         {
           "metric": {

We should add a CI check to make sure fixtures are updated before merging so irrelevant changes don't get added later

[damemi]

fixed by #460