[gdb/testsuite] Fix gdb.python/py-selected-context.exp with glibc debuginfo

With test-case gdb.python/py-selected-context.exp, I run into:
...
(gdb) thread 1
[Switching to thread 1.1 (Thread 0x7ffff7cc0080 (LWP 1793477))]
57          return INTERNAL_SYSCALL_CANCEL (...
Python Exception <class 'gdb.GdbError'>: error from gdb_selected_context_handler
(gdb) FAIL: $exp: switch thread, handler raises an error
...

The regexp doesn't expect the source line, which indeed is not printed if I
de-install glibc debuginfo.

Fix this by updating the regexp.

Tested on x86_64-linux.