Added tests and did improved class and file structure.

tests/formats/test_id3.py

@@ -0,0 +1,30 @@
+#!/usr/bin/python3
+
+from smalltag import SmallTag
+
+
+# chars = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789-_ "
+
+# no "b" because there is a bug in TinyTag 2.1.0 that removes "b"s if they are at the beginning of a string
+chars = "acdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789- _"
+
+
+def test_writing_text_frames():
+    for char1 in chars:
+        for char2 in chars:
+            field_content = f"{char1}{char2}Test{char2}{char1}"
+
+            tag = SmallTag.get("formats/test.mp3")
+
+            tag.title = field_content
+            tag.artist = field_content
+            tag.album = field_content
+
+            tag.write()
+
+            tag = SmallTag.get("formats/test.mp3")
+
+            assert tag.title == field_content
+            assert tag.artist == field_content
+            assert tag.album == field_content
+