tests: Test for explicit start/end args to str methods for unicode.

tests/unicode/unicode_pos.py

@@ -0,0 +1,5 @@
+# str methods with explicit start/end pos
+print("Привет".startswith("П"))
+print("Привет".startswith("р", 1))
+print("абвба".find("а", 1))
+print("абвба".find("а", 1, -1))