build(deps): bump robolectric from 4.7.3 to 4.8

Bumps [robolectric](https://github.com/robolectric/robolectric) from 4.7.3 to 4.8.
- [Release notes](https://github.com/robolectric/robolectric/releases)
- [Commits](robolectric/robolectric@robolectric-4.7.3...robolectric-4.8)

---
updated-dependencies:
- dependency-name: org.robolectric:robolectric
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

rsdroid/build.gradle

@@ -65,7 +65,7 @@ dependencies {
     implementation 'com.jakewharton.timber:timber:5.0.1'
 
     testImplementation 'junit:junit:4.13.2'
-    testImplementation "org.robolectric:robolectric:4.7.3"
+    testImplementation "org.robolectric:robolectric:4.8"
     testImplementation 'androidx.test:core:1.4.0'
     testImplementation "androidx.test.ext:junit:${rootProject.ext.androidxTestJunitVersion}"
     testImplementation project(path: ':rsdroid-testing')