Harden tests' bash

Use `set -u` and `set -o pipefail` to catch accidental mistakes and
failures more strongly.

 - `set -u` catches the use of undefined variables
 - `set -o pipefail` catches failures (like `set -e`) earlier in the
   pipeline.

This makes the tests a bit more robust. It is nice to read code not
worrying about these spurious success paths (via uncaught) errors
undermining the tests. Indeed, I caught some bugs doing this.

There are a few tests where we run a command that should fail, and then
search its output to make sure the failure message is one that we
expect. Before, since the `grep` was the last command in the pipeline
the exit code of those failing programs was silently ignored. Now with
`set -o pipefail` it won't be, and we have to do something so the
expected failure doesn't accidentally fail the test.

To do that we use `expect` and a new `expectStderr` to check for the
exact failing exit code. See the comments on each for why.

`grep -q` is replaced with `grepQuiet`, see the comments on that
function for why.

`grep -v` when we just want the exit code is replaced with `grepInverse,
see the comments on that function for why.

`grep -q -v` together is, surprise surprise, replaced with
`grepQuietInverse`, which is both combined.

Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>

tests/db-migration.sh

@@ -2,7 +2,7 @@
 
 # Only run this if we have an older Nix available
 # XXX: This assumes that the `daemon` package is older than the `client` one
-if [[ -z "$NIX_DAEMON_PACKAGE" ]]; then
+if [[ -z "${NIX_DAEMON_PACKAGE-}" ]]; then
     exit 99
 fi
 
@@ -12,7 +12,7 @@ killDaemon
 
 # Fill the db using the older Nix
 PATH_WITH_NEW_NIX="$PATH"
-export PATH="$NIX_DAEMON_PACKAGE/bin:$PATH"
+export PATH="${NIX_DAEMON_PACKAGE}/bin:$PATH"
 clearStore
 nix-build simple.nix --no-out-link
 nix-store --generate-binary-cache-key cache1.example.org $TEST_ROOT/sk1 $TEST_ROOT/pk1