Fix: export-ignore: tests/ => /tests

It's much better to use:

    /tests export-ignore

than

    tests/ export-ignore

The rule:

    tests/ export-ignore

doesn't work on Windows and it probably covers all subdirectories. If we want to cover all subdirectories why not use:

    tests export-ignore

Anyway, the rule:

    /tests export-ignore

works both on unix and windows and it covers only one directory (the one from root dir).