CI: Add a test coverage job

This uses grcov internally to generate the HTML report.  We use grcov
instead of, say, lcov, because grcov actually generates accessible
pages.

We use the gnome-build-meta:core-nightly container image.  It doesn't
contain grcov by default, but we download the binary from its github
page.  We can change this later to build a custom container image for
gnome-calendar's own CI.

This will publish the report on

  https://gnome.pages.gitlab.gnome.org/gnome-calendar/coverage/

.gitlab-ci/build-with-coverage.sh

@@ -0,0 +1,9 @@
+#!/bin/sh
+
+set -eux -o pipefail
+
+export CFLAGS="-coverage -ftest-coverage -fprofile-arcs"
+
+meson setup --buildtype=debug --prefix /usr _build .
+meson compile -C _build
+meson test -C _build