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/test/java/com/keylesspalace/tusky/SpanUtilsTest.kt

@@ -136,7 +136,7 @@ class SpanUtilsTest {
         }
 
         override fun <T : Any> getSpans(start: Int, end: Int, type: Class<T>): Array<T> {
-            return spans.filter { it.start >= start && it.end <= end && type.isInstance(it) }
+            return spans.filter { it.start >= start && it.end <= end && type.isInstance(it.span) }
                 .map { it.span }
                 .toTypedArray() as Array<T>
         }