Fix state dependency in TestSuite

[edsko]

Currently we still have to run the tests without reusing sessions, because we don't fully manage to reset session state in between tests. Partly this is due to #224, but I tihnk there are also still some other issues. This is a problem for the TestSuite because it slows it down a lot, but it's also a problem for ide-backend itself (indeed, I found #224 precisely because I was trying to fix this problem, and may find other issues in a similar way).

[edsko]

Part of the state dependency seems to be another instance of #189. I have looked at that in quite some detail but I cannot figure what is going on. It seems to be that when we get redundant "Compiling such-and-such" messages they come from the linking step; Cabal first calls ghc to compile everything and then separately calls it again to link. What seems to happen is that ghc detects at the linking step that "flags have changed" and decides to recompile some modules again, even though it just compiled those in the previous step; I tried to print the flags that it is actually comparing, and I can't see the differences -- but it is comparing fingerprints, one of which is loaded from the module interface, so I am probably comparing the wrong thing, although I cannot see quite where I'm going wrong. I can't make sense of it. For now am I just going to relax the tests so that they allow for the additional progress message; it's not big deal with ghc does recompile a module one more time on buildExe I suppose, nor is it disastrous for the IDE to show that compiling message twice. I would like to understand better why this is happening but I guess it's not a high priority.

[edsko]

Part of the problem is #238.

[edsko]

There still seem to be two problems left: #256 and #257, neither of which is high priority though because they affect the TestSuite only (hopefully), although not entirely sure about #257.