PYTHON-3303 Upgrade encryption testing to macos 10.15+ (#1004)

Author: juliusgeo

.evergreen/config.yml

@@ -1843,6 +1843,14 @@ axes:
           skip_ECS_auth_test: true
           python3_binary: /Library/Frameworks/Python.framework/Versions/3.8/bin/python3
           libmongocrypt_url: https://s3.amazonaws.com/mciuploads/libmongocrypt/macos/master/latest/libmongocrypt.tar.gz
+      - id: macos-1100
+        display_name: "macOS 11.00"
+        run_on: macos-1100
+        variables:
+          skip_EC2_auth_test: true
+          skip_ECS_auth_test: true
+          python3_binary: /Library/Frameworks/Python.framework/Versions/3.8/bin/python3
+          libmongocrypt_url: https://s3.amazonaws.com/mciuploads/libmongocrypt/macos/master/latest/libmongocrypt.tar.gz
       - id: rhel62
         display_name: "RHEL 6.2 (x86_64)"
         run_on: rhel62-small
@@ -2287,7 +2295,7 @@ buildvariants:
 - matrix_name: "test-macos-encryption"
   matrix_spec:
     platform:
-      - macos-1014
+      - macos-1100
     auth: "auth"
     ssl: "nossl"
     encryption: "*"

test/test_encryption.py

@@ -1777,10 +1777,9 @@ def setUp(self):
         self.cipher_text = self.client_encryption.encrypt(
             "hello", key_id=keyID, algorithm=Algorithm.AEAD_AES_256_CBC_HMAC_SHA_512_Deterministic
         )
-        if self.cipher_text[-1] == 0:
-            self.malformed_cipher_text = self.cipher_text[:-1] + b"1"
-        else:
-            self.malformed_cipher_text = self.cipher_text[:-1] + b"0"
+        self.malformed_cipher_text = self.cipher_text[:-1] + (self.cipher_text[-1] ^ 1).to_bytes(
+            1, "big"
+        )
         self.malformed_cipher_text = Binary(self.malformed_cipher_text, 6)
         opts = AutoEncryptionOpts(
             key_vault_namespace="keyvault.datakeys", kms_providers=kms_providers_map