[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.

gdb/testsuite/gdb.tui/corefile-run.exp

@@ -29,11 +29,7 @@ if { [prepare_for_testing "failed to prepare" $testfile $srcfile debug] } {
 }
 
 # Only run on native boards.
-require !use_gdb_stub
-if { [target_info gdb_protocol] == "extended-remote" } {
-    untested "not supported"
-    return
-}
+require {target_can_use_run_cmd core}
 
 if { ![runto_main] } {
     return -1

gdb/testsuite/lib/gdb.exp

@@ -385,13 +385,33 @@ proc delete_breakpoints {} {
 
 # Returns true iff the target supports using the "run" command.
 
-proc target_can_use_run_cmd {} {
+proc target_can_use_run_cmd { {target_description ""} } {
+    if { $target_description == "" } {
+	set have_core 0
+    } elseif { $target_description == "core" } {
+	# We could try to figure this out by issuing an "info target" and
+	# checking for "Local core dump file:", but it would mean the proc
+	# would start requiring a current target. Also, uses while gdb
+	# produces non-standard output due to, say annotations would
+	# have to be moved around or eliminated, which would further limit
+	# usability.
+	set have_core 1
+    } else {
+	error "invalid argument: $target_description"
+    }
+
     if [target_info exists use_gdb_stub] {
 	# In this case, when we connect, the inferior is already
 	# running.
 	return 0
     }
 
+    if { $have_core && [target_info gdb_protocol] == "extended-remote" } {
+	# In this case, when we connect, the inferior is not running but
+	# cannot be made to run.
+	return 0
+    }
+
     # Assume yes.
     return 1
 }