Cleanup test skipping

- Try not to put cryptic "99" in many places

  Factor out `exit 99` into `skipTest` function

- Alows make sure skipping a test is done with a reason

  `skipTest` takes a mandatory argument

- Separate pure conditionals vs side-effectful test skipping.

  "require daemon" already had this, but "sandbox support" did not.

tests/linux-sandbox.sh

@@ -4,13 +4,13 @@ needLocalStore "the sandbox only runs on the builder side, so it makes no sense
 
 clearStore
 
-if ! canUseSandbox; then exit 99; fi
+requireSandboxSupport
 
 # Note: we need to bind-mount $SHELL into the chroot. Currently we
 # only support the case where $SHELL is in the Nix store, because
 # otherwise things get complicated (e.g. if it's in /bin, do we need
 # /lib as well?).
-if [[ ! $SHELL =~ /nix/store ]]; then exit 99; fi
+if [[ ! $SHELL =~ /nix/store ]]; then skipTest "Shell is not from Nix store"; fi
 
 chmod -R u+w $TEST_ROOT/store0 || true
 rm -rf $TEST_ROOT/store0