Tests: use TEST_NGINX_BINARY to find nginx binary.

If not specified, fallback to ../nginx/objs/nginx.

lib/Test/Nginx.pm

diff --git a/lib/Test/Nginx.pm b/lib/Test/Nginx.pm
index 244f881..1e2884a 100644
--- a/lib/Test/Nginx.pm
+++ b/lib/Test/Nginx.pm
@@ -22,7 +22,8 @@
 
 ###############################################################################
 
-our $NGINX = '../nginx/objs/nginx';
+our $NGINX = defined $ENV{TEST_NGINX_BINARY} ? $ENV{TEST_NGINX_BINARY}
+	: '../nginx/objs/nginx';
 
 sub new {
 	my $self = {};