User search: provide 10 results not 5

Also refactor slightly so the caller specifies the number of
results, which lets all the constants sit in the repository.

libraries/usersearch/test/src/main/kotlin/io/element/android/libraries/usersearch/test/FakeUserListDataSource.kt

@@ -25,7 +25,7 @@ class FakeUserListDataSource : UserListDataSource {
     private var searchResult: List<MatrixUser> = emptyList()
     private var profile: MatrixUser? = null
 
-    override suspend fun search(query: String): List<MatrixUser> = searchResult
+    override suspend fun search(query: String, count: Long): List<MatrixUser> = searchResult.take(count.toInt())
 
     override suspend fun getProfile(userId: UserId): MatrixUser? = profile