[gdb/testsuite] Require ![is_remote host] for TUI

When running test-case gdb.tui/corefile-run.exp with both host and target board
local-remote-host-native.exp, we run into:
...
FAIL: gdb.tui/corefile-run.exp: load corefile
...
while this passes with USE_TUI=0.

The problem is that the TUI setup code uses "setenv TERM ansi", which has no
effect on remote host.

I can confirm this analysis by working around this problem in
local-remote-host-native.exp like this:
...
-    spawn $RSH -t -l $username $remote $cmd
+    spawn $RSH -t -l $username $remote "export TERM=ansi; $cmd"
...

For now, simply make TUI unsupported for remote host, by returning 0 in
prepare_for_tui.

Tested on x86_64-linux.

gdb/testsuite/lib/tuiterm.exp

@@ -792,6 +792,14 @@ namespace eval Term {
     # Setup ready for starting the tui, but don't actually start it.
     # Returns 1 on success, 0 if TUI tests should be skipped.
     proc prepare_for_tui {} {
+	if { [is_remote host] } {
+	    # In clean_restart, we're using "setenv TERM ansi", which has
+	    # effect on build.  If we have [is_remote host] == 0, so
+	    # build == host, then it also has effect on host.  But for
+	    # [is_remote host] == 1, it has no effect on host.
+	    return 0
+	}
+
 	if {![allow_tui_tests]} {
 	    return 0
 	}