tests: Ensure failure if beakerlib results file not found

In example here
https://209.132.184.41:8493/logs/pull-887-20191025-100505-509b9eae-weldr-lorax--fedora-31-tar/log.html

we're seeing stuff like:
/tests/test_cli.sh: line 109: /tmp/composer-test.wIAITH/TestResults: No such file or directory

but the result is PASS

Cherry-picked from 2b578dcee8

Related: rhbz#1770193

tests/cli/lib/lib.sh

@@ -37,6 +37,9 @@ run_beakerlib_tests() {
 }
 
 parse_beakerlib_results() {
+    if [ ! -f "$BEAKERLIB_DIR/TestResults" ]; then
+        exit "$BEAKERLIB_DIR/TestResults not found" 1
+    fi
     . $BEAKERLIB_DIR/TestResults
 
     TESTRESULT_RESULT_ECODE="${TESTRESULT_RESULT_ECODE:-}"