test: make SessionOutput Promise tolerant

If a test case would use Promises for linesUntil, sometimes we would
emit lines events while there was no listener attached to it. This
commit changes SessionOutput to make it a little more resilient: now
line events will only be emitted when the SessionOutput is waiting. When
wait is called (via linesUntil, for example), it'll retrieve all lines
(via line events) on the buffer until it reaches a line matching the
regexp.  If no lines match the regexp, we'll continue as before waiting
for data from lldb and buffering it.

PR-URL: #311
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

test/common.js

@@ -45,10 +45,13 @@ function SessionOutput(session, stream, timeout) {
   this.timeout = timeout || 10000;
   this.session = session;
 
-  stream.on('data', (data) => {
-    buf += data;
-
+  this.flush = function flush() {
     for (;;) {
+      // NOTE(mmarchini): don't emit line events while not waiting, otherwise
+      // we might miss something.
+      if (!this.waiting)
+        break
+
       let index = buf.indexOf('\n');
 
       if (index === -1)
@@ -62,6 +65,11 @@ function SessionOutput(session, stream, timeout) {
       else
         this.emit('line', line);
     }
+  }
+
+  stream.on('data', (data) => {
+    buf += data;
+    this.flush();
   });
 
   // Ignore errors
@@ -129,6 +137,7 @@ SessionOutput.prototype.wait = function wait(regexp, callback, allLines) {
   }, this.timeout).unref();
 
   this.on('line', onLine);
+  this.flush();
 };
 
 SessionOutput.prototype.waitBreak = function waitBreak(callback) {