tests/fuzzers: fix goroutine leak in les fuzzer (#22455)

The oss-fuzz fuzzer has been reporting some failing testcases for les. They're all spurious, and cannot reliably be reproduced. However, running them showed that there was a goroutine leak: the tests created a lot of new clients, which started an exec queue that was never torn down.

This PR fixes the goroutine leak, and also a log message which was erroneously formatted.