[thread-cert] misc enhancements to improve reliability (#7298)

This commit introduces a number of enhancements to `thread-cert`
Border Router tests to improve reliability:
- Checks `ot-rcp` process status for OTBR tests to help catch errors
  earlier
- Make sure `ot-rcp` processes are terminated after one test to avoid
  corrupting subsequent tests
- Upload core dump as artifacts if otbr-agent crashed

.github/workflows/otbr.yml

@@ -93,6 +93,8 @@ jobs:
           *.pcap
           *.json
           *.log
+          coredump_*
+          otbr-agent_*
     - name: Generate Coverage
       run: |
         ./script/test generate_coverage gcc
@@ -162,6 +164,8 @@ jobs:
           *.pcap
           *.json
           *.log
+          coredump_*
+          otbr-agent_*
     - name: Generate Coverage
       run: |
         ./script/test generate_coverage gcc