Fix bogus "Scan interrupted" scanTime in mass-testing JSON (#1246)

In mass-testing mode main() runs run_mass_testing()/run_mass_testing_parallel()
and exits without ever calling lets_roll() for the scan itself, so
calc_scantime() never runs and SCAN_TIME stays 0. fileout_json_footer(),
invoked from cleanup() on exit, treats SCAN_TIME==0 as an interrupted scan
and writes "scanTime": "Scan interrupted" for the whole batch.

START_TIME is already set by the earlier "lets_roll init" call, so compute
the overall scan time with calc_scantime() before exiting the mass-testing
path. The aggregate footer now reports the real elapsed time as an INFO
finding instead of a spurious WARN.

testssl.sh

@@ -26291,7 +26291,12 @@ lets_roll() {
           else
                run_mass_testing
           fi
-          exit $?
+          RET=$?
+          # START_TIME was set by "lets_roll init" above; compute the overall scan
+          # time here so fileout_json_footer() doesn't mistake it for an interrupted
+          # scan (SCAN_TIME==0) and report "Scan interrupted", see #1246
+          calc_scantime
+          exit $RET
      fi
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