Add tests for StreamEntriesHelper#rtl? (#2286)

We used some random Arabic characters to test that the various RTL
conditions got triggered.

spec/helpers/stream_entries_helper_spec.rb

@@ -14,4 +14,22 @@ RSpec.describe StreamEntriesHelper, type: :helper do
       expect(helper.display_name(account)).to eq "Username"
     end
   end
+
+  describe '#rtl?' do
+    it 'is false if text is empty' do
+      expect(helper).not_to be_rtl ''
+    end
+
+    it 'is false if there are no right to left characters' do
+      expect(helper).not_to be_rtl 'hello world'
+    end
+
+    it 'is false if right to left characters are fewer than 1/3 of total text' do
+      expect(helper).not_to be_rtl 'hello ݟ world'
+    end
+
+    it 'is true if right to left characters are greater than 1/3 of total text' do
+      expect(helper).to be_rtl 'aaݟ'
+    end
+  end
 end