ingest/ledgerbackend: Captive core's LogLineWriter should be closed synchronously

[2opremio]

We just close the writer side of the pipe but don't wait for the goroutine to exit:

go/ingest/ledgerbackend/stellar_core_runner.go

Lines 197 to 209 in a4e5a3f

	func (r *stellarCoreRunner) getLogLineWriter() io.Writer {
	rd, wr := io.Pipe()
	br := bufio.NewReader(rd)
	// Strip timestamps from log lines from captive stellar-core. We emit our own.
	dateRx := regexp.MustCompile(`^\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}\.\d{3} `)
	go func() {
	levelRx := regexp.MustCompile(`\[(\w+) ([A-Z]+)\] (.*)`)
	for {
	line, err := br.ReadString('\n')
	if err != nil {
	break
	}

This can cause an extra log line to be written after closing the backend, which causes problems in the new Soroban RPC integration tests. See https://github.com/stellar/soroban-rpc/actions/runs/9538438244/job/26287637351?pr=216#step:10:50

In this particular case, from a dangling goroutine is causing a race condition with Go's testing library.