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/fetchClosure.sh

@@ -42,13 +42,13 @@ if [[ "$NIX_REMOTE" != "daemon" ]]; then
 fi
 
 # 'toPath' set to empty string should fail but print the expected path.
-nix eval -v --json --expr "
+expectStderr 1 nix eval -v --json --expr "
   builtins.fetchClosure {
     fromStore = \"file://$cacheDir\";
     fromPath = $nonCaPath;
     toPath = \"\";
   }
-" 2>&1 | grep "error: rewriting.*$nonCaPath.*yielded.*$caPath"
+" | grep "error: rewriting.*$nonCaPath.*yielded.*$caPath"
 
 # If fromPath is CA, then toPath isn't needed.
 nix copy --to file://$cacheDir $caPath