Fix flakiness in umask spec test.

The test asserts that there is no stderr, but this depended on whether
the temp files existed.

Addresses issue #22.

spec/builtins.test.sh

@@ -209,7 +209,7 @@ umask | grep '[0-9]\+'  # check for digits
## status: 0

#### set umask in octal
rm $TMP/umask-one $TMP/umask-two
rm -f $TMP/umask-one $TMP/umask-two
umask 0002
echo one > $TMP/umask-one
umask 0022