Group dependabot updates for some gradle dependencies (#8100)

After #8048 I was expecting dependabot to update all packages given a particular dependency bump, but that doesn't seem to be happening.  For example #8097 only updated shared_preferences_android.

So let's try adding [`groups`](https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file#groups) to see if that does it.  Group some dependencies together.

Fixes flutter/flutter#148098

Author: jmagman

.github/dependabot.yml

@@ -105,6 +105,20 @@ updates:
     open-pull-requests-limit: 10
     labels:
       - "autosubmit"
+    groups:
+      test-dependencies:
+        patterns:
+          - "androidx.test:*"
+          - "io.mockk:mockk:*"
+          - "junit:junit"
+          - "org.mockito:*"
+          - "org.robolectric:*"
+      gradle-plugin:
+        patterns:
+          - "org.jetbrains.kotlin:kotlin-gradle-plugin"
+      androidx:
+        patterns:
+          - "androidx.annotation:annotation"
     ignore:
       - dependency-name: "com.android.tools.build:gradle"
         update-types: ["version-update:semver-minor", "version-update:semver-patch"]