[gdb/testsuite] Fix error message for cmd with trailing newline

I noticed that the error message in gdb_test_multiple about trailing newline
in a command does not mention the offending command, nor the word command:
...
    if [string match "*\[\r\n\]" $command] {
        error "Invalid trailing newline in \"$message\" test"
    }
...

Fix this by using instead:
...
        error "Invalid trailing newline in \"$command\" command"
...

Also add a test-case to trigger this: gdb.testsuite/gdb-test.exp.

Tested on x86_64-linux.

gdb/testsuite/gdb.testsuite/gdb-test.exp

@@ -0,0 +1,28 @@
+# Copyright 2022 Free Software Foundation, Inc.
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# The purpose of this test-case is to check various
+# gdb_test / gdb_test_multiple properties.
+
+clean_restart
+
+# Check that a command with trailing newline triggers an error.
+
+set results [catch {
+    gdb_test "pwd\n" ".*" "cmd with trailing newline"
+} output]
+
+gdb_assert { $results == 1 }
+set expected_error_msg "Invalid trailing newline in \"pwd\n\" command"
+gdb_assert { [string equal $output $expected_error_msg] }

gdb/testsuite/lib/gdb.exp

@@ -1000,7 +1000,7 @@ proc gdb_test_multiple { command message args } {
     }
 
     if [string match "*\[\r\n\]" $command] {
-	error "Invalid trailing newline in \"$message\" test"
+	error "Invalid trailing newline in \"$command\" command"
     }
 
     if [string match "*\[\r\n\]*" $message] {