Add test coverage support with kcov

This MR adds support for test coverage to be displayed by gitlab. Coverage is generated with kcov and the resulting cobertura report uploaded as artifact.

I based pretty much everything on the excellent work already done in dbscripts, I however took the liberty of:

• collapsing test and coverage in the same step, so that we only run bats once
• adding a coverage regex in the pipeline, for coverage results to be parsed and displayed in gitlab's ui

Covered lines will show up for new MRs, and in the job's ui as percent covered:

.gitlab-ci.yml

 stages:
     - check
 
-check:
-    stage: check
+.pacman_install:
     before_script:
         # NOTE: Install latest archlinux-keyring before upgrading system. In the
         # future this should not be needed anymore when we can guarantee a valid
         # keyring for longer:
         # https://gitlab.archlinux.org/archlinux/archlinux-keyring/-/issues/4
-        - echo -e "\e[0Ksection_start:`date +%s`:pacman[collapsed=true]\r\e[0KInstalling ShellCheck"
+        - echo -e "\e[0Ksection_start:`date +%s`:pacman[collapsed=true]\r\e[0KInstalling dependencies"
         - pacman -Sy --needed --noconfirm archlinux-keyring
-        - pacman --noconfirm -Syu --needed git make shellcheck
+        - pacman --noconfirm -Syu --needed bash-bats bash-bats-assert bash-bats-support git jq kcov lzop make shellcheck
         - echo -e "\e[0Ksection_end:`date +%s`:pacman\r\e[0K"
+
+check:
+    stage: check
+    extends: .pacman_install
     script:
         - make shellcheck
 
 test:
     stage: check
-    before_script:
-        - echo -e "\e[0Ksection_start:`date +%s`:pacman[collapsed=true]\r\e[0KInstalling bats"
-        - pacman -Sy --needed --noconfirm archlinux-keyring
-        - pacman --noconfirm -Syu --needed git make bash-bats bash-bats-assert bash-bats-support lzop
-        - echo -e "\e[0Ksection_end:`date +%s`:pacman\r\e[0K"
+    extends: .pacman_install
     script:
         - make --silent check BATS_ARGS='-F junit' > bats-report.xml
     artifacts:
         reports:
             junit: bats-report.xml
+
+coverage:
+    stage: check
+    extends: .pacman_install
+    coverage: '/Percent covered\s+\d+\.\d+/'
+    script:
+        - make coverage 2>/dev/null
+        - jq -r '. |
+            ["Percent covered", .percent_covered],
+            ["Covered lines", .covered_lines],
+            ["Total lines", .total_lines],
+            ["Percent low", .percent_low],
+            ["Percent high", .percent_high] | @tsv' coverage/bats.*/coverage.json
+    artifacts:
+        reports:
+            coverage_report:
+                coverage_format: cobertura
+                path: coverage/bats.*/cobertura.xml