Disable tests and benchs by default on new-install

[fgaz]

And error if they are explicitly enabled, so we never try to install
remote packages with tests and/or benchs.

Fixes #4855

Please include the following checklist in your PR:

• Patches conform to the coding conventions.
• Any changes that could be relevant to users have been recorded in the changelog.
• The documentation has been updated, if necessary.
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Please also shortly describe how you tested your change. Bonus points for added tests!
...no tests for new-install yet

[fgaz]

It's the second time that this happens https://travis-ci.org/haskell-pushbot/cabal-binaries/builds/299703001#L655

    ShortText Id:                                                                         FAIL

      *** Failed! Falsifiable (after 100 tests and 4 shrinks): 

      "\65534"

      Use --quickcheck-replay=209006 to reproduce.

Again the U+FFFE "not a character" (reversed BOM)

[23Skidoo]

It's a known issue, I think we had a ticket. You can ignore it. /cc @hvr

[23Skidoo]

OK, let's merge. CI errors are spurious.

[23Skidoo]

Merged, thanks!