[nix-local-build] Packages without tests should not considered --enable-tests as part of configuration

[ezyang]

Steps to reproduce:

1. Create a Cabal package with no test suite
2. cabal new-build
3. cabal new-build --enable-tests

Expected behavior: Cabal reports nothing to do

Actual behavior: Cabal decides to rebuild the package, because configuration has changed

CC @dcoutts

[ezyang]

I was going to try to fix this myself, but after having reviewed the ProjectPlanning code related to tests, I must admit I have no idea what is going on. There is some logic about "sticky flags" and I don't understand how this interacts with @dcoutt's claims here #3316 (comment) :

The only time you really need to specify --enable-tests is if the solver was not able to optimistically enable the testsuite.

So why do we need the extra stickiness?

UPDATE: I understand what is going on, writing a comment for #3316.