Fix testing index when /bin/sh is dash (like on Debian)

Contrary to bash, dash filters out environment variables
containing '-' characters.

src/zc/buildout/easy_install.py

@@ -62,7 +62,7 @@ except ImportError:
     from urlparse import urljoin
 
 default_index_url = os.environ.get(
-    'buildout-testing-index-url',
+    'BUILDOUT_TESTING_INDEX_URL',
     'https://pypi.org/simple',
     )
 

src/zc/buildout/testing.py

@@ -230,7 +230,7 @@ def buildoutSetUp(test):
     register_teardown(lambda: rmtree(tmp))
 
     zc.buildout.easy_install.default_index_url = 'file://'+tmp
-    os.environ['buildout-testing-index-url'] = (
+    os.environ['BUILDOUT_TESTING_INDEX_URL'] = (
         zc.buildout.easy_install.default_index_url)
 
     def tmpdir(name):

src/zc/buildout/tests.py

@@ -3428,7 +3428,7 @@ def easy_install_SetUp(test):
 def buildout_txt_setup(test):
     zc.buildout.testing.buildoutSetUp(test)
     mkdir = test.globs['mkdir']
-    eggs = os.environ['buildout-testing-index-url'][7:]
+    eggs = os.environ['BUILDOUT_TESTING_INDEX_URL'][7:]
     test.globs['sample_eggs'] = eggs
     create_sample_eggs(test)