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/string_fstring_invalid.py

@@ -0,0 +1,13 @@
+# PEP-0498 specifies that '\\' and '#' must be disallowed explicitly, whereas
+# MicroPython relies on the syntax error as a result of the substitution.
+
+print(f"\\")
+print(f"#")
+try:
+    eval("f'{\}'")
+except SyntaxError:
+    print("SyntaxError")
+try:
+    eval("f'{#}'")
+except SyntaxError:
+    print("SyntaxError")