Add markdown test summaries and a coverage HTML report to CI

ctest_summary.py renders a Test.xml as markdown for
GITHUB_STEP_SUMMARY: a one-liner when everything passes, otherwise the
first few failures inline with a link to the full Test.xml on MinIO.
It's also usable locally with --all to list every failure from a
downloaded Test.xml.

The coverage job now also generates and uploads gcovr's html-details
report and links it from the step summary.

ctest_summary.py

@@ -0,0 +1,76 @@
+#!/usr/bin/env python3
+"""Summarize a CTest Test.xml as markdown.
+
+Intended for $GITHUB_STEP_SUMMARY in CI, where only the first few failures
+are shown inline (pass --link to point at the full Test.xml). Also reusable
+locally to print every failure from a downloaded Test.xml with --all.
+"""
+
+import argparse
+import base64
+import gzip
+import xml.etree.ElementTree as ET
+
+# Failure output is truncated to this many trailing characters, which is
+# usually enough to include e.g. an ASan report's summary.
+OUTPUT_TAIL_CHARS = 3000
+
+
+def test_output(test):
+    value = test.find("./Results/Measurement/Value")
+    if value is None or value.text is None:
+        return ""
+    text = value.text
+    if value.get("encoding") == "base64":
+        raw = base64.b64decode(text)
+        if value.get("compression") == "gzip":
+            raw = gzip.decompress(raw)
+        text = raw.decode(errors="replace")
+    return text
+
+
+def main():
+    parser = argparse.ArgumentParser(description=__doc__)
+    parser.add_argument("test_xml", help="path to a ctest Testing/*/Test.xml")
+    parser.add_argument(
+        "--inline",
+        type=int,
+        default=5,
+        help="how many failures to show inline (default 5)",
+    )
+    parser.add_argument("--all", action="store_true", help="show every failure inline")
+    parser.add_argument(
+        "--link", help="URL of the full Test.xml, linked when failures are elided"
+    )
+    args = parser.parse_args()
+
+    testing = ET.parse(args.test_xml).getroot().find("Testing")
+    tests = testing.findall("Test")
+    failed = [t for t in tests if t.get("Status") == "failed"]
+    notrun = sum(1 for t in tests if t.get("Status") == "notrun")
+
+    if not failed:
+        print(f"✅ All {len(tests) - notrun} tests passed")
+    else:
+        print(f"❌ {len(failed)} of {len(tests)} tests failed\n")
+        shown = failed if args.all else failed[: args.inline]
+        for test in shown:
+            name = test.findtext("Name")
+            output = test_output(test)[-OUTPUT_TAIL_CHARS:].strip()
+            print(f"<details><summary><code>{name}</code></summary>\n")
+            print("````")
+            print(output)
+            print("````")
+            print("</details>\n")
+        remaining = len(failed) - len(shown)
+        if remaining > 0:
+            more = f"… and {remaining} more"
+            if args.link:
+                more += f" — full list in [Test.xml]({args.link})"
+            print(more)
+    if notrun:
+        print(f"\n⚠️ {notrun} tests not run")
+
+
+if __name__ == "__main__":
+    main()