test: make varlink StartTransient checks compatible with jq 1.6

test: make varlink StartTransient checks compatible with jq 1.6

The new "varlinkctl --more StartTransient" subtest pipes a JSON-SEQ
stream of multiple records into "jq --seq -e ...". CentOS 9
ships jq 1.6, where -e only inspects the last input record's output:
when the trailing record (the final reply) doesn't match the
"select()" filter, jq exits non-zero even though earlier records
match, so the test fails.

Use --slurp which collapses the records into an array first and
returns a single bool.

Follow-up for 1cde1cc3bab595fe7b4e2befbfa08a01a172db0f