t7006: guard cleanup with test_expect_success

Most of these tests are removing files, environment variables, and
configuration that might interfere outside the test.  Putting these
clean-up commands in the test (in the same spirit as v1.7.1-rc0~59,
2010-03-20) means that errors during setup will be caught quickly and
non-error text will be suppressed without -v.

While at it, apply some other minor fixes:

 - do not rely on the shell to export variables defined with the same
   command as a function call

 - avoid whitespace immediately after the > redirection operator, for
   consistency with the style of other tests

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/test-lib.sh

@@ -512,6 +512,22 @@ test_must_fail () {
 	test $? -gt 0 -a $? -le 129 -o $? -gt 192
 }
 
+# Similar to test_must_fail, but tolerates success, too.  This is
+# meant to be used in contexts like:
+#
+#	test_expect_success 'some command works without configuration' '
+#		test_might_fail git config --unset all.configuration &&
+#		do something
+#	'
+#
+# Writing "git config --unset all.configuration || :" would be wrong,
+# because we want to notice if it fails due to segv.
+
+test_might_fail () {
+	"$@"
+	test $? -ge 0 -a $? -le 129 -o $? -gt 192
+}
+
 # test_cmp is a helper function to compare actual and expected output.
 # You can use it like:
 #