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>

tests/basics/string_fstring_debug.py.exp

@@ -0,0 +1,6 @@
+x=1
+x=00000001
+a x=1 b 2 c
+a x=00000001 b 2 c
+a f() + g("foo") + h()=15 b
+a f() + g("foo") + h()=0000000f b