Make CI catch more errors

Running ./validate --legacy in master (6ff89c17), I got

Stage 1:

Unexpected failures:
   /run/user/1000/ghctest-1q4jw5ja/test   spaces/T2632.run  T2632 [exit code non-0] (normal)

Stage 2:

Unexpected failures:
   /run/user/1000/ghctest-x7g95s52/test   spaces/haddock/haddock_testsuite/haddockHoogleTest.run  haddockHoogleTest [bad exit code (2)] (normal)
   /run/user/1000/ghctest-x7g95s52/test   spaces/haddock/haddock_testsuite/haddockHtmlTest.run    haddockHtmlTest [bad exit code (2)] (normal)
   /run/user/1000/ghctest-x7g95s52/test   spaces/haddock/haddock_testsuite/haddockHypsrcTest.run  haddockHypsrcTest [bad exit code (2)] (normal)
   /run/user/1000/ghctest-x7g95s52/test   spaces/haddock/haddock_testsuite/haddockLatexTest.run   haddockLatexTest [bad exit code (2)] (normal)
   /run/user/1000/ghctest-x7g95s52/test   spaces/driver/T16318/T16318.run                         T16318 [bad exit code (2)] (normal)
   /run/user/1000/ghctest-x7g95s52/test   spaces/hpc/T17073.run                                   T17073 [bad stdout] (hpc)
   /run/user/1000/ghctest-x7g95s52/test   spaces/driver/T18125/T18125.run                         T18125 [bad exit code (2)] (normal)

The errors are fixed in !3776 (closed).

The goal of this ticket is to improve CI to catch issues like this.

Used environment: NixOS 20.03 + ghc.nix.