tests/basics/string_fstring.py: Test fstring nested replacement fields.

This does not actually cover any additional lines, but it does cover new
functionality not previously covered.

Signed-off-by: Jeff Epler <[email protected]>

Author: jepler

tests/basics/string_fstring.py

@@ -79,3 +79,14 @@ def foo(a, b):
 # Raw f-strings.
 print(rf"\r\a\w {'f'} \s\t\r\i\n\g")
 print(fr"\r{x}")
+
+# Format specifiers with nested replacement fields
+space = 5
+prec = 2
+print(f"{3.14:{space}.{prec}}")
+
+space_prec = "5.2"
+print(f"{3.14:{space_prec}}")
+
+radix = "x"
+print(f"{314:{radix}}")