tests: Add S/MIME messages to protected-headers corpus

These sample messages are taken directly from the Protected Headers
draft:

https://www.ietf.org/id/draft-autocrypt-lamps-protected-headers-02.html

Note that this commit doesn't strictly pass the common git pre-commit
hook due to introducing some trailing whitespace.  That's just the
nature of the corpus, though.  We should have that trailing
whitespace, so I've made this commit with --no-verify.

Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>

test/corpora/protected-headers/smime-enc+legacy-disp.eml

@@ -0,0 +1,50 @@
+Received: from localhost (localhost [127.0.0.1]); Wed, 27 Nov 2019
+ 01:27:28 -0700 (UTC-07:00)
+MIME-Version: 1.0
+Content-Transfer-Encoding: base64
+Content-Type: application/pkcs7-mime; name="smime.p7m";
+ smime-type="enveloped-data"
+From: Alice Lovelace <alice@smime.example>
+To: Bob Babbage <bob@smime.example>
+Date: Wed, 27 Nov 2019 01:27:00 -0700
+Message-ID: <smime-enc+legacy-disp@protected-headers.example>
+Subject: ...
+
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+

test/corpora/protected-headers/smime-multipart-signed.eml

@@ -0,0 +1,68 @@
+Received: from localhost (localhost [127.0.0.1]); Tue, 26 Nov 2019
+ 20:03:17 -0400 (UTC-04:00)
+MIME-Version: 1.0
+Content-Type: multipart/signed; boundary="179";
+ protocol="application/pkcs7-signature"; micalg="sha-256"
+From: Alice Lovelace <alice@smime.example>
+To: Bob Babbage <bob@smime.example>
+Date: Tue, 26 Nov 2019 20:03:00 -0400
+Subject: The FooCorp contract
+Message-ID: <smime-multipart-signed@protected-headers.example>
+
+--179
+Content-Type: text/plain; charset="us-ascii"; protected-headers="v1"
+From: Alice Lovelace <alice@smime.example>
+To: Bob Babbage <bob@smime.example>
+Date: Tue, 26 Nov 2019 20:03:00 -0400
+Subject: The FooCorp contract
+Message-ID: <smime-multipart-signed@protected-headers.example>
+
+Bob, we need to cancel this contract.
+
+Please start the necessary processes to make that happen today.
+
+(this is the 'smime-multipart-signed' message)
+
+Thanks, Alice
+-- 
+Alice Lovelace
+President
+Example Corp
+
+--179
+Content-Transfer-Encoding: base64
+Content-Type: application/pkcs7-signature; name="smime.p7s"
+
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+
+--179--
+

test/corpora/protected-headers/smime-onepart-signed.eml

@@ -0,0 +1,54 @@
+Received: from localhost (localhost [127.0.0.1]); Tue, 26 Nov 2019
+ 20:06:17 -0400 (UTC-04:00)
+Content-Transfer-Encoding: base64
+Content-Type: application/pkcs7-mime; name="smime.p7m";
+ smime-type="signed-data"
+MIME-Version: 1.0
+From: Alice Lovelace <alice@smime.example>
+To: Bob Babbage <bob@smime.example>
+Date: Tue, 26 Nov 2019 20:06:00 -0400
+Subject: The FooCorp contract
+Message-ID: <smime-onepart-signed@protected-headers.example>
+
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+

test/corpora/protected-headers/smime-sign+enc+legacy-disp.eml

@@ -0,0 +1,102 @@
+Received: from localhost (localhost [127.0.0.1]); Wed, 27 Nov 2019
+ 01:24:28 -0700 (UTC-07:00)
+MIME-Version: 1.0
+Content-Transfer-Encoding: base64
+Content-Type: application/pkcs7-mime; name="smime.p7m";
+ smime-type="enveloped-data"
+From: Alice Lovelace <alice@smime.example>
+To: Bob Babbage <bob@smime.example>
+Date: Wed, 27 Nov 2019 01:24:00 -0700
+Message-ID: <smime-sign+enc+legacy-disp@protected-headers.example>
+Subject: ...
+
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test/corpora/protected-headers/smime-sign+enc.eml

@@ -0,0 +1,95 @@
+Received: from localhost (localhost [127.0.0.1]); Wed, 27 Nov 2019
+ 01:15:28 -0700 (UTC-07:00)
+MIME-Version: 1.0
+Content-Transfer-Encoding: base64
+Content-Type: application/pkcs7-mime; name="smime.p7m";
+ smime-type="enveloped-data"
+From: Alice Lovelace <alice@smime.example>
+To: Bob Babbage <bob@smime.example>
+Date: Wed, 27 Nov 2019 01:15:00 -0700
+Message-ID: <smime-sign+enc@protected-headers.example>
+Subject: ...
+
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