test: fail testing if network port is busy

Commit "test: handle everything with go test"
(9a6159a) handles everything except a
case when tests are not terminated when Tarantool start is failed due to
busy network port. Patch adds a dumb check that make sure desired port
is not busy.

Part of #147

test_helpers/main.go

@@ -14,10 +14,12 @@ import (
 	"errors"
 	"fmt"
 	"log"
+	"net"
 	"os"
 	"os/exec"
 	"regexp"
 	"strconv"
+	"strings"
 	"time"
 
 	"github.com/tarantool/go-tarantool"
@@ -185,6 +187,13 @@ func StartTarantool(startOpts StartOpts) (TarantoolInstance, error) {
 		return inst, err
 	}
 
+	netOpts := strings.Split(startOpts.Listen, ":")
+	ln, err := net.Listen("tcp", ":"+netOpts[1])
+	if err != nil {
+		return inst, err
+	}
+	ln.Close()
+
 	// Options for restarting tarantool instance.
 	inst.Opts = startOpts