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

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.

	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
	}