[processing] handle Processing.TEMPORARY_OUTPUT for folder parameters

[alexbruy]

Description

Follow up #8979. Seems we missed that QgsProcessingParameterFolderDestination also needs to handle temporaty outputs.

Checklist

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[alexbruy]

@nyalldawson not sure that this is the best solution, probably we need to introduce special method for directory outputs, like we already have for files.

[nyalldawson]

Looks good, can we get a unit test covering this?

[alexbruy]

@nyalldawson on the second though, we can use parameterAsFileOutput() for directories to retrieve path. So looks like this fix is not really necessary, we just need to fix few algorithms by replacing parameterAsString() with parameterAsFileOutput() where needed.

I tend to close this and use parameterAsFileOutput() to get directory path.

[nyalldawson]

@alexbruy I think we need this PR and it's safer to continue here -- there's potentially external plugins using parameterAsString for this which would have broken with the change.

[alexbruy]

@nyalldawson not 100% sure, but seems there is a typo here and temporary file was never resolved to the correct path. Fixed it too, when adding test for temporary directory output.

[alexbruy]

Test added. Also seems we missed a typo in the #8979, so fixed it too.

[nyalldawson]

Thanks!