[gdb/testsuite] Remove f-string in gdb.python/py-unwind.py

on openSUSE Leap 42.3, with python 3.4, I run into a
"SyntaxError: invalid syntax" due to usage of an f-string in test-case
gdb.python/py-unwind.py.

Fix this by using string concatenation using '+' instead.

Tested on x86_64-linux.