Add test for byte string slicing.

tests/basics/tests/slice-bstr1.py

+b"123"[0:1]
+
+b"123"[0:2]
+
+b"123"[:1]
+
+b"123"[1:]
+
+# Idiom for copying sequence
+b"123"[:]
+
+b"123"[:-1]
+
+# Weird cases
+b"123"[0:0]
+b"123"[1:0]
+b"123"[1:1]
+b"123"[-1:-1]
+b"123"[-3:]
+b"123"[-3:3]
+b"123"[0:]
+b"123"[:0]
+b"123"[:-3]
+b"123"[:-4]
+# No IndexError!
+b""[1:1]
+b""[-1:-1]