tests: Add test for file.seek().

tests/io/file_seek.py

+f = open("io/data/file1", "rb")
+print(f.seek(6))
+print(f.read(5))
+
+print(f.seek(0, 1))
+print(f.read(4))
+
+print(f.seek(-6, 2))
+print(f.read(20))
+
+print(f.seek(0, 0))
+print(f.read(5))