[gimp/gimp-2-8] tests: fix running make check with both parallel make and xvfb

[Michael Henning <mhenning src gnome org>]

commit ecb06953ef22e4532b9a449d9d0694452fc3bba9
Author: Michael Henning <drawoc darkrefraction com>
Date:   Sun May 12 19:36:27 2013 -0400

    tests: fix running make check with both parallel make and xvfb
    
    Before, the xvfbs would all try to be on DISPLAY=:99, which
    caused issues.
    (cherry picked from commit 2df4fa2d6f3c75b6e88589dc356f5682ca6f9624)

 app/tests/Makefile.am |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/app/tests/Makefile.am b/app/tests/Makefile.am
index b9e1937..821b76c 100644
--- a/app/tests/Makefile.am
+++ b/app/tests/Makefile.am
@@ -14,7 +14,7 @@ TESTS_ENVIRONMENT = \
 
 # Run tests with xvfb-run if available
 if HAVE_XVFB_RUN
-TESTS_ENVIRONMENT += $(XVFB_RUN) --server-args="-screen 0 1280x1024x24"
+TESTS_ENVIRONMENT += $(XVFB_RUN) --auto-servernum --server-args="-screen 0 1280x1024x24"
 endif