Changed the way the test setup avoids reading ~/.buildout/default.cfg

to work with recent change changes to use expanduser rather than
reading $HOME directly.

src/zc/buildout/buildout.txt

@@ -1127,6 +1127,7 @@ reading the configuration file.  ($HOME is the value of the HOME
 environment variable. The '/' is replaced by the operating system file
 delimiter.)
 
+    >>> old_home = os.environ['HOME']
     >>> home = tmpdir('home')
     >>> mkdir(home, '.buildout')
     >>> write(home, '.buildout', 'default.cfg',
@@ -1167,7 +1168,7 @@ user defaults:
     op5 b3base 5
     recipe recipes:debug
 
-    >>> del os.environ['HOME']
+    >>> os.environ['HOME'] = old_home
 
 Log level
 ---------

src/zc/buildout/testing.py

@@ -156,13 +156,19 @@ def buildoutSetUp(test):
     here = os.getcwd()
     register_teardown(lambda: os.chdir(here))
 
-    old_home = os.environ.get('HOME')
-    if old_home is not None:
-        del os.environ['HOME'] # pop doesn't truly remove it :(
-        register_teardown(lambda: os.environ.__setitem__('HOME', old_home))
 
     base = tempfile.mkdtemp('buildoutSetUp')
     register_teardown(lambda base=base: shutil.rmtree(base))
+
+    old_home = os.environ.get('HOME')
+    os.environ['HOME'] = os.path.join(base, 'bbbBadHome')
+    def restore_home():
+        if old_home is None:
+            del os.environ['HOME']
+        else:
+            os.environ['HOME'] = old_home
+    register_teardown(restore_home)
+
     base = os.path.join(base, '_TEST_')
     os.mkdir(base)