[gdb/testsuite] Further handle long filenames in gdb.base/startup-with-shell.exp

[gdb/testsuite] Further handle long filenames in gdb.base/startup-with-shell.exp

In commit 52498004a34 ("gdb/testsuite: handle long filenames in
gdb.base/startup-with-shell.exp") we fixed a FAIL reported by the Linaro CI:
...
(gdb) print argv[1]
$1 = 0xfffed978 "<snip>/startup-with-shell/unique-file.unique-e"...
(gdb) FAIL: gdb.base/startup-with-shell.exp: startup_with_shell = on; \
  run_args = *.unique-extension: first argument expanded
...

PR testsuite/31410 reports a similar failure:
...
(gdb) print argv[1]
$1 = 0xfffeda96 "<snip>/startup-with-shell/*.unique-extens"...
(gdb) FAIL: gdb.base/startup-with-shell.exp: startup_with_shell = off; \
  run_args = *.unique-extension: first argument not expanded
...

Fix this in the same way, using "set print characters unlimited".

Tested on x86_64-linux.

Approved-By: Tom Tromey <tom@tromey.com>
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31410