tests: Add some tests to improve coverage.

tests/unicode/data/utf-8_2.txt

@@ -1 +1,2 @@
 aαbβcγdδ
+ぁ🙐

tests/unicode/file2.py

@@ -10,6 +10,16 @@ def do(mode):
     print(f.read(1))
     print(f.read(2))
     print(f.read(4))
+
+    # skip to end of line
+    f.readline()
+
+    # check 3-byte utf-8 char
+    print(f.read(1 if mode == 'rt' else 3))
+
+    # check 4-byte utf-8 char
+    print(f.read(1 if mode == 'rt' else 4))
+
     f.close()
 
 do('rb')