Test Fspec in regression for bug prevention

[stefjoosten]

In issue #1380 we discovered that it would be useful to do an equality check on the data structure FSpec in the regression, to detect a large category of possible deep mistakes. For example, this would have saved us issue #1380 in the first place.

A complication is that FSpec contains functions. So this is not entirely straightforward. Hence this issue, so we won't forget.