test/stages/users: make test data date agnostic

The test for this stage is failing because etc/shadow changes content
depending on the date that it runs on (due to the "date of last password
change" field). This causes the checksums to not be constant for our
tests and depends on the date.

This commit removes the checksums for etc/shadow from the test so that
they are not checked as part of the test. This worksaround the test
failure issue for now until a solution to the dynamic contents is
determined.

Signed-off-by: Michael Ho <michael.ho@ieee.org>

test/data/stages/users/diff.json

@@ -80,14 +80,14 @@
     },
     "/etc/shadow": {
       "content": [
-        "sha256:61aa8fa91f7055a95b8a4a20dbaed2981fed58b7ef5e82acc5a863aa96fb660a",
-        "sha256:9ef562601c7a940f2ff402d47459d2a3987ac433edc5b31cb45fd9a571a955c2"
+        null,
+        null
       ]
     },
     "/etc/shadow-": {
       "content": [
-        "sha256:f567820b1e89fc27f089f3b2342e443f7ede4b2468e31ece83dfe7961f547c9e",
-        "sha256:39852bf1a919afcff2188b3cc28f17176f77582907f4b87ab9946f47eb9f2b41"
+        null,
+        null
       ]
     },
     "/etc/subgid": {