[gdb/testsuite] Use regular proc syntax for gdb_caching_proc

A regular tcl proc with no args looks like:
...
proc foo {} {
     return 1
}
...
but a gdb_caching_proc deviates from that syntax by dropping the explicit no
args bit:
...
gdb_caching_proc foo {
     return 1
}
...

Make the gdb_caching_proc use the same syntax as regular procs, such that we
have instead:
...
gdb_caching_proc foo {} {
     return 1
}
...

Tested on x86_64-linux.

Reviewed-By: Tom Tromey <tom@tromey.com>