[Coverage] Don't produce a JUnit report.

The `unittest` binary crashes with
```
unittest: /var/lib/gitlab-runner/builds/yS6csq8A/0/bigdata/mutable/mutable/third-party/catch2/include/catch2/catch.hpp:5918: virtual void Catch::CumulativeReporterBase<Catch::JunitReporter>::testCaseEnded(const Catch::TestCaseStats &) [DerivedT = Catch::JunitReporter]: Assertion `m_sectionStack.size() == 0' failed.
```
This error seems to be related to the Catch 2 JUnit reporter.  See
catchorg/Catch2#1801 and
catchorg/Catch2#1967.

To remedy this problem, we simply don't use produce a report anymore.
It was never used, anyway.

.gitlab-ci.yml

@@ -402,11 +402,6 @@ report:coverage:
         - .coverage
         - .shared
         - .build_cache
-    artifacts:
-        paths:
-            - "build/${BUILD_TYPE}/catch.xml"
-        reports:
-            junit: "build/${BUILD_TYPE}/catch.xml"
     rules:
         - if: $CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH
           when: on_success
@@ -600,4 +595,4 @@ publish:github-release:
         -H "X-GitHub-Api-Version: 2022-11-28"
         -H "Content-Type: application/octet-stream"
         "${upload_url}?name=mutable-macos-amd64-${TAG}.zip"
-        --data-binary "@mutable/mutable-macos-amd64.zip"
+        --data-binary "@mutable/mutable-macos-amd64.zip"

coverage.sh

@@ -59,7 +59,7 @@ lcov ${LCOV_FLAGS} --zerocounters --directory src --directory unittest
 lcov ${LCOV_FLAGS} --capture --initial --directory src --directory unittest --output-file base.info
 # lcov ${LCOV_FLAGS} --capture --initial --output-file unittest.base
 # Run tests
-bin/unittest --durations yes --reporter junit --out catch.xml '[core]'
+bin/unittest --reporter compact '[core]'
 # Capture lcov counters and generate report
 lcov ${LCOV_FLAGS} --capture --directory src --directory unittest --output-file test.info
 # lcov ${LCOV_FLAGS} --capture --directory unittest --output-file unittest.capture