Hadrian should respect the build root setting for the testsuite and its own binaries

Currently the --build-root setting is respected only partially:

• The test rule appears to be placing all testsuite build artefacts right in the GHC tree ./testsuite.
• Hadrian puts its own binaries also in the GHC tree, into the ./hadrian/X directory, where X depends on whether we build with Cabal or Stack.

We should improve on this and provide more guarantees about where all build artefacts go.

Trac metadata

Trac field	Value
Version	8.6.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Build System (Hadrian)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	alpmestan, simonpj
Operating system
Architecture