Ignore "@instance..." part of username when computing status length (#3392)

* Move compose.* tests to own namespace

* Ignore "@instance..." part of username when computing status length

In a status with a mention ("@foo@example.org") only the "@foo" part should
be included in the calculated status length. It wasn't, so the app was
prevening people from posting statuses that should have been allowed.

Fix this.

- Lift the length calculation code in to a separate static function (easier
  and faster to test)
- Add a `MentionSpan` type, to reuse existing code for detecting mentions
- Fix a bug in `FakeSpannable.getSpans()` (it was returning the outer type,
  not the wrapped inner span)
- Add additional fast tests

The tests made sense under the `components.compose.ComposeActivity` package,
so I also created that and moved the existing ComposeActivity tests there.

Fixes https://github.com/tuskyapp/Tusky/issues/3339

* Static import assertEquals

app/src/main/java/com/keylesspalace/tusky/util/LinkHelper.kt

@@ -156,7 +156,7 @@ private fun getCustomSpanForMention(mentions: List<Mention>, span: URLSpan, list
 }
 
 private fun getCustomSpanForMentionUrl(url: String, mentionId: String, listener: LinkListener): ClickableSpan {
-    return object : NoUnderlineURLSpan(url) {
+    return object : MentionSpan(url) {
         override fun onClick(view: View) = listener.onViewAccount(mentionId)
     }
 }