ci: Fix FIH YAML result parsing

The running test was generating an YAML output with the following line:

```
last_line: '  Description: 'ECDSA signature test of attest token''
```

This string comes from the test with the single quotes, and using single
quotes twice break the YAML format, so this commit changes the string
after last_line to be enclosed in the double quotes.

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/ci/fih_test_docker/fi_tester_gdb.sh b/ci/fih_test_docker/fi_tester_gdb.sh
index 8a5dbb0..2a20a1d 100755
--- a/ci/fih_test_docker/fi_tester_gdb.sh
+++ b/ci/fih_test_docker/fi_tester_gdb.sh
@@ -104,7 +104,7 @@
                 echo "    test_exec_ok: True"
                 echo "    skipped: True"
                 echo "    boot: False"
-                echo "    last_line: '$LAST_LINE' "
+                echo "    last_line: \"$LAST_LINE\" "
             fi
         else
             # The target was not stopped at the desired address.