Store multiple invocations of dmesg check for the same test in one file

This should prevent loosing dmesg reports in the case of tests that
restart or reboot: all reports except the last one are lost.

The filename remains the same, but it's no longer overwritten. `dmesg`
check invocations append to it, with proper headers:

```
\## Acquired at 2024-12-03T13:12:23.536502+00:00
[    0.000000] Linux version 6.11.4-301.fc41.x86_64 (mockbuild@9b6b61418589428cb880a7020233b56f) (gcc (GCC) 14.2.1 20240912 (Red Hat 14.2.1-3), GNU ld version 2.43.1-2.fc41) #1 SMP PREEMPT_DYNAMIC Sun Oct 20 15:02:33 UTC 2024
[    0.000000] Command line: BOOT_IMAGE=(hd0,gpt3)/vmlinuz-6.11.4-301.fc41.x86_64 no_timer_check console=tty1 console=ttyS0,115200n8 systemd.firstboot=off root=UUID=815e66c2-6a8a-4984-a890-1a3c710bf933 rootflags=subvol=root
[    0.000000] BIOS-provided physical RAM map:
[    0.000000] BIOS-e820: [mem 0x0000000000000000-0x000000000009fbff] usable
...
\## Acquired at 2024-12-03T13:12:23.596179+00:00
...
\## Acquired at 2024-12-03T13:12:23.622283+00:00
...
```

Fixes #3317

tests/test/check/data/main.fmf

@@ -13,6 +13,15 @@
             failure-pattern:
               - 'Hypervisor detected'
 
+    /multiple-reports:
+        check:
+          - how: dmesg
+            result: respect
+          - how: dmesg
+            result: respect
+          - how: dmesg
+            result: respect
+
 /avc:
   check:
     - how: avc

tests/test/check/test-dmesg.sh

@@ -20,6 +20,7 @@ rlJournalStart
         rlRun "harmless=$run/plan/execute/data/guest/default-0/dmesg/harmless-1"
         rlRun "segfault=$run/plan/execute/data/guest/default-0/dmesg/segfault-1"
         rlRun "custom_patterns=$run/plan/execute/data/guest/default-0/dmesg/custom-patterns-1"
+        rlRun "multiple_reports=$run/plan/execute/data/guest/default-0/dmesg/multiple-reports-1"
 
         rlRun "pushd data"
         rlRun "set -o pipefail"
@@ -97,6 +98,22 @@ rlJournalStart
             rlAssertExists "$dump_after"
             rlLogInfo "$(cat $dump_after)"
         rlPhaseEnd
+
+        rlPhaseStartTest "Test multiple dmesg reports with $PROVISION_HOW"
+            rlRun "dump_before=$multiple_reports/checks/dmesg-before-test.txt"
+            rlRun "dump_after=$multiple_reports/checks/dmesg-after-test.txt"
+
+            rlRun "tmt run --id $run --scratch -a -vv provision -h $PROVISION_HOW test -n /dmesg/multiple-reports"
+            rlRun "cat $results"
+
+            rlLogInfo "$(cat $dump_before)"
+            rlAssertEquals "There should be 3 reports before the test" \
+                           "$(grep 'Acquired at' $dump_before | wc -l)" "3"
+
+            rlLogInfo "$(cat $dump_after)"
+            rlAssertEquals "There should be 3 reports after the test" \
+                           "$(grep 'Acquired at' $dump_after | wc -l)" "3"
+        rlPhaseEnd
     fi
 
     rlPhaseStartCleanup

tmt/checks/dmesg.py

@@ -11,7 +11,12 @@
 from tmt.checks import Check, CheckEvent, CheckPlugin, _RawCheck, provides_check
 from tmt.result import CheckResult, ResultOutcome
 from tmt.steps.provision import GuestCapability
-from tmt.utils import Path, field, format_timestamp, render_run_exception_streams
+from tmt.utils import (
+    Path,
+    field,
+    format_timestamp,
+    render_command_report,
+    )
 
 if TYPE_CHECKING:
     import tmt.base
@@ -102,21 +107,22 @@ def _save_dmesg(
         path = invocation.check_files_path / TEST_POST_DMESG_FILENAME.format(event=event.value)
 
         try:
-            dmesg_output = self._fetch_dmesg(invocation.guest, logger)
+            output = self._fetch_dmesg(invocation.guest, logger)
 
         except tmt.utils.RunError as exc:
             outcome = ResultOutcome.ERROR
-            output = "\n".join(render_run_exception_streams(exc.output, verbose=1))
+            output = exc.output
 
         else:
             outcome = ResultOutcome.PASS
-            output = dmesg_output.stdout or ''
-            if any(pattern.search(output) for pattern in self.failure_pattern):
+
+            if any(pattern.search(output.stdout or '') for pattern in self.failure_pattern):
                 outcome = ResultOutcome.FAIL
 
         invocation.phase.write(
             path,
-            f'# Acquired at {timestamp}\n{output}')
+            '\n'.join(render_command_report(label=f'Acquired at {timestamp}', output=output)),
+            mode='a')
 
         return outcome, path.relative_to(invocation.phase.step.workdir)