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/suite/perfschema/t/statement_program_lost_inst.test

@@ -19,10 +19,10 @@
 --source include/have_perfschema.inc
 
 let $restart_file= $MYSQLTEST_VARDIR/tmp/mysqld.1.expect;
---exec echo "wait" > $restart_file
+--write_line wait $restart_file
 --shutdown_server 
 --source include/wait_until_disconnected.inc
---exec echo "restart:--performance_schema_max_program_instances=7 --performance_schema_max_statement_stack=2 --thread_stack=655360">$restart_file
+--write_line "restart:--performance_schema_max_program_instances=7 --performance_schema_max_statement_stack=2 --thread_stack=655360" $restart_file
 --enable_reconnect
 --source include/wait_until_connected_again.inc