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.py

@@ -12,17 +12,11 @@ print(f"""no interpolation""")
 x, y = 1, 2
 print(f'{x}')
 print(f'{x:08x}')
-print(f'{x=}')
-print(f'{x=:08x}')
 print(f'a {x} b {y} c')
 print(f'a {x:08x} b {y} c')
-print(f'a {x=} b {y} c')
-print(f'a {x=:08x} b {y} c')
 
 print(f'a {"hello"} b')
 print(f'a {f() + g("foo") + h()} b')
-print(f'a {f() + g("foo") + h()=} b')
-print(f'a {f() + g("foo") + h()=:08x} b')
 
 def foo(a, b):
     return f'{x}{y}{a}{b}'