[gdb/testsuite] Fix untested message in gdb.tui/corefile-run.exp

In test-case gdb.tui/corefile-run.exp, we have this bit:
...
require !use_gdb_stub
if { [target_info gdb_protocol] == "extended-remote" } {
    untested "not supported"
    return
}
...

So with target board native-gdbserver we get:
...
UNSUPPORTED: gdb.tui/corefile-run.exp: require failed: !use_gdb_stub
...
and with target board native-extended-gdbserver instead:
...
UNTESTED: gdb.tui/corefile-run.exp: not supported
...

Fix this by:
- adding an optional argument target_description to proc
  target_can_use_run_cmd
- handling the target_description == core &&
  [target_info gdb_protocol] == "extended-remote" case in the proc
- using require {target_can_use_run_cmd core}
such that now in both cases we have:
...
UNSUPPORTED: gdb.tui/corefile-run.exp: require failed: \
  target_can_use_run_cmd core
...

Tested on x86_64-linux.