Tests: handled early pidfile write on win32 in the run() routine.

In addition to the present waiting for pidfile, which is insufficient on win32
due to the CreateProcess model, and may lead to rare startup races, search now
for the certain error message which indicates started worker process.

This change allows tolerating moderate hiccups on win32 hosts.
diff --git a/lib/Test/Nginx.pm b/lib/Test/Nginx.pm
index c3772f0..b51b8b6 100644
--- a/lib/Test/Nginx.pm
+++ b/lib/Test/Nginx.pm
@@ -345,6 +345,12 @@
 	$self->waitforfile("$testdir/nginx.pid", $pid)
 		or die "Can't start nginx";
 
+	for (1 .. 50) {
+		last if $^O ne 'MSWin32';
+		last if $self->read_file('error.log') =~ /create thread/;
+		select undef, undef, undef, 0.1;
+	}
+
 	$self->{_started} = 1;
 	return $self;
 }