Allow packages to omit AvailabilityTest in their PackageInfo.g

[fingolfin]

If missing we assume it is ReturnTrue which is what >= 100 of our packages use.

[ThomasBreuer]

The change from lib/grpfpiso.gi should better be proposed in another pull request.

Concerning the failures because some AvailabilityTest functions return something different from true or false, eventually the functions should be fixed, but for the moment we can continue accepting these results.

[fingolfin]

@ThomasBreuer fixed now

[ThomasBreuer]

Thanks.
Now just the outputs tested in tst/testinstall/package.tst must be adjusted:
The line complaining about the missing AvailabilityTest must be removed from the output of ValidatePackageInfo(rec());.
Perhaps also the AvailabilityTest components can be removed from the later info records (or perhaps be replaced by a non-function).

[fingolfin]

Adjusted the validator tests as suggested, and then cleaned them up a bit more for good measure.