[gdb/testsuite] Fix gdb.python/py-source-styling-2.exp with TERM=dumb

When running test-case gdb.python/py-source-styling-2.exp with TERM=dumb, I
get:
...
(gdb) set style enabled on^M
warning: The current terminal doesn't support styling. \
  Styled output might not appear as expected.^M
(gdb) FAIL: $exp: set style enabled on
...

Fix this by using with_ansi_styling_terminal on clean_restart.

Tested on x86_64-linux.