Set <pkgname_datadir> to an absolute path

[mpickering]

Ticket #10717 points out that the _datadir environment variable
should be set to an absolute path since the test executables may
change directory during their invocation.

This is easily fixed by using the absoluteWorkingDirLBI
function, which was introduced in 027bddf,

Fixes #10717

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[Mikolaj]

I take it you deem this ready for review, so let me add the label for that.

[mpickering]

@ulysses4ever I have put up a backport on #10838. I don't plan to make any more changes to the backport, so the person preparing the release can decide what a suitable course of action is and modify that patch or this patch as they deem necessary.

[ulysses4ever]

Sounds good, thanks @mpickering !

[Mikolaj]

Apparently, conflicts prevent a merge.

BTW, should this be backported to 3.14? [Edit: it already is, in a different form.]

[ulysses4ever]

It looks like it was my inline comment above (#10830 (comment)) that was holding it back. Resolving...