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/main/mysql_client_test_comp.test

@@ -12,7 +12,7 @@ SET @old_slow_query_log= @@global.slow_query_log;
 
 call mtr.add_suppression(" Error reading file './client_test_db/test_frm_bug.frm'");
 call mtr.add_suppression(" IP address .* could not be resolved");
---exec echo "$MYSQL_CLIENT_TEST" > $MYSQLTEST_VARDIR/log/mysql_client_test_comp.out.log 2>&1
+--write_line "$MYSQL_CLIENT_TEST" $MYSQLTEST_VARDIR/log/mysql_client_test_comp.out.log
 --exec $MYSQL_CLIENT_TEST --getopt-ll-test=25600M >> $MYSQLTEST_VARDIR/log/mysql_client_test_comp.out.log 2>&1
 
 # End of test