tests/basics: Split f-string debug printing to separate file with .exp.

This feature {x=} was introduced in Python 3.8 so needs a separate .exp
file to run on earlier Python versions.

See https://bugs.python.org/issue36817

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

tools/tinytest-codegen.py

@@ -101,6 +101,7 @@ exclude_tests = (
     "misc/sys_settrace_features.py",
     # don't have f-string
     "basics/string_fstring.py",
+    "basics/string_fstring_debug.py",
 )
 
 output = []