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/gen-coverage.sh

@@ -0,0 +1,14 @@
+#!/bin/sh
+
+set -eux -o pipefail
+
+mkdir -p public
+grcov _build --source-dir ./ --prefix-dir ../ --output-type cobertura --branch --ignore-not-existing -o public/coverage.xml
+grcov _build --source-dir ./ --prefix-dir ../ --output-type html --branch --ignore-not-existing -o public/coverage
+
+# Print "Coverage: 42.42" so .gitlab-ci.yml will pick it up with a regex
+#
+# We scrape this from the HTML report, not the JSON summary, because coverage.json
+# uses no decimal places, just something like "42%".
+
+grep -Eo 'abbr title.* %' public/coverage/index.html | head -n 1 | grep -Eo '[0-9.]+ %' | grep -Eo '[0-9.]+' | awk '{ print "Coverage:", $1 }'