tests/basics: Move str/bytes tests that give SyntaxWarning to sep file.

In CPython 3.12 these invalid str/bytes/fstring escapes will issue a
SyntaxWarning, and so differ to MicroPython.

Signed-off-by: Damien George <damien@micropython.org>

tests/basics/string1.py

@@ -5,7 +5,6 @@ print('abc')
 print(r'abc')
 print(u'abc')
 print(repr('\a\b\t\n\v\f\r'))
-print('\z') # unrecognised escape char
 
 # construction
 print(str())