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/fetchGitRefs.sh

@@ -56,7 +56,7 @@ invalid_ref() {
     else
         (! git check-ref-format --branch "$1" >/dev/null 2>&1)
     fi
-    nix --debug eval --raw --impure --expr "(builtins.fetchGit { url = $repo; ref = ''$1''; }).outPath" 2>&1 | grep 'invalid Git branch/tag name' >/dev/null
+    expect 1 nix --debug eval --raw --impure --expr "(builtins.fetchGit { url = $repo; ref = ''$1''; }).outPath" 2>&1 | grep 'invalid Git branch/tag name' >/dev/null
 }