MDEV-16944 Fix file sharing issues on Windows in mysqltest

On Windows systems, occurrences of ERROR_SHARING_VIOLATION due to
conflicting share modes between processes accessing the same file can
result in CreateFile failures.

mysys' my_open() already incorporates a workaround by implementing
wait/retry logic on Windows.

But this does not help if files are opened using shell redirection like
mysqltest traditionally did it, i.e via

--echo exec "some text" > output_file

In such cases, it is cmd.exe, that opens the output_file, and it
won't do any sharing-violation retries.

This commit addresses the issue by introducing a new built-in command,
'write_line', in mysqltest. This new command serves as a brief alternative
to 'write_file', with a single line output, that also resolves variables
like "exec" would.

Internally, this command will use my_open(), and therefore retry-on-error
logic.

Hopefully this will eliminate the very sporadic "can't open file because
it is used by another process" error on CI.

mysql-test/include/start_mysqld.inc

@@ -21,7 +21,7 @@ if ($restart_bindir)
 
 if ($restart_parameters)
 {
-  --exec echo "$restart_cmd: $restart_parameters" > $_expect_file_name
+  --write_line "$restart_cmd: $restart_parameters" $_expect_file_name
   if (!$restart_noprint)
   {
     --replace_result $MYSQL_TEST_DIR MYSQL_TEST_DIR $MYSQLTEST_VARDIR MYSQLTEST_VARDIR
@@ -34,7 +34,7 @@ if ($restart_parameters)
 }
 if (!$restart_parameters)
 {
-  --exec echo "$restart_cmd" > $_expect_file_name
+  --write_line "$restart_cmd" $_expect_file_name
   if ($restart_noprint < 2)
   {
     --exec echo "# $restart_cmd"