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/function-trace.sh

@@ -10,17 +10,15 @@ expect_trace() {
             --trace-function-calls \
             --expr "$expr" 2>&1 \
             | grep "function-trace" \
-            | sed -e 's/ [0-9]*$//'
+            | sed -e 's/ [0-9]*$//' \
+            || true
     )
 
     echo -n "Tracing expression '$expr'"
-    set +e
     msg=$(diff -swB \
                <(echo "$expect") \
                <(echo "$actual")
-    );
-    result=$?
-    set -e
+    ) && result=0 || result=$?
     if [ $result -eq 0 ]; then
         echo " ok."
     else
@@ -67,5 +65,3 @@ expect_trace '1 2' "
 function-trace entered «string»:1:1 at
 function-trace exited «string»:1:1 at
 "
-
-set -e