test failures on 32 bit architectures.

[David Bremner]

I know, I know, I don't use 32 bit architectures either. However...

Looking at [1], it looks like there two tests consistently failing on 32
bit architectures (and also mips64el, FWIW).

    T160-json: Testing --format=json output
     FAIL   Search message: json, 64-bit timestamp
            --- T160-json.8.expected	2020-06-22 12:29:35.053363072 +0000
            +++ T160-json.8.output	2020-06-22 12:29:35.053363072 +0000
            @@ -1,7 +1,7 @@
             [
                 {
                     "authors": "Notmuch Test Suite",
            -        "date_relative": "the future",
            +        "date_relative": "1970-01-01",
                     "matched": 1,
                     "query": [
                         "id:msg-005 at notmuch-test-suite",
            @@ -13,7 +13,7 @@
                         "unread"
                     ],
                     "thread": "XXX",
            -        "timestamp": 32472187200,
            +        "timestamp": 0,
                     "total": 1
                 }
             ]

    T355-smime: Testing S/MIME signature verification and decryption
     FAIL   Verify signature on PKCS#7 SignedData message
            expires: value not equal: data[0][0][0]["crypto"]["signed"]["status"][0]["expires"] = 2145914603 != 2611032858

I haven't tried to debug yet, but the first bug looks like a timestamp
overflowing 32 bits, becoming negative, and being clamped to 0.
No idea about the second.

[1]: https://buildd.debian.org/status/package.php?p=notmuch&suite=experimental