chore (OONI Run V2): Add missing error message when no tests are sele…

…cted (#700)

- Fixes  ooni/run#142

## Proposed Changes

  - Add `Toast` where no test is selected
 
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![Screenshot_20240320_173337](https://github.com/ooni/probe-android/assets/17911892/ee2bb49d-7411-4807-91b0-1a2445aa5ba4)
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![Screenshot_20240320_173411](https://github.com/ooni/probe-android/assets/17911892/b4c4986a-f608-483a-b8c4-4223e9892f4b)
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app/src/main/java/org/openobservatory/ooniprobe/activity/runtests/RunTestsActivity.kt

@@ -4,6 +4,7 @@ import android.content.Context
 import android.content.Intent
 import android.os.Bundle
 import android.text.TextUtils
+import android.widget.Toast
 import androidx.core.content.ContextCompat
 import androidx.core.graphics.drawable.DrawableCompat
 import org.openobservatory.engine.BaseNettest
@@ -103,6 +104,8 @@ class RunTestsActivity : AbstractActivity() {
  { finish() },
  preferenceManager
  )
+ } else {
+ Toast.makeText(this@RunTestsActivity, "Please select test to run", Toast.LENGTH_LONG).show()
  }
  }