Tests: reworked try_run() to redirect stderr.

Previously, if TEST_NGINX_VERBOSE was set, the code tried to use compiled-in
error log.  While this works in typical development cases when --error-log-path
is relative to prefix, it is unlikely to work correctly if --error-log-path
is absolute.

The recently introduced '-e' command line option might be used to fix this.
On the other hand, a more compatible solution would be to redirect stderr
to a file instead.

lib/Test/Nginx.pm

diff --git a/lib/Test/Nginx.pm b/lib/Test/Nginx.pm
index 5ebfa2a..fa3d102 100644
--- a/lib/Test/Nginx.pm
+++ b/lib/Test/Nginx.pm
@@ -291,19 +291,24 @@
 	my ($self, $message) = @_;
 
 	eval {
-		open OLDERR, ">&", \*STDERR; close STDERR;
+		open OLDERR, ">&", \*STDERR;
+		open NEWERR, ">", $self->{_testdir} . '/stderr'
+			or die "Can't open stderr: $!";
+		close STDERR;
+		open STDERR, ">&", \*NEWERR;
+		close NEWERR;
+
 		$self->run();
+
+		close STDERR;
 		open STDERR, ">&", \*OLDERR;
 	};
 
 	return $self unless $@;
 
 	if ($ENV{TEST_NGINX_VERBOSE}) {
-		my $path = $self->{_configure_args} =~ m!--error-log-path=(\S+)!
-			? $1 : 'logs/error.log';
-		$path = "$self->{_testdir}/$path" if index($path, '/');
-
-		open F, '<', $path or die "Can't open $path: $!";
+		open F, '<', $self->{_testdir} . '/stderr'
+			or die "Can't open stderr: $!";
 		log_core($_) while (<F>);
 		close F;
 	}