Stop using hardcoded cert fingerprint in test_08_Puller_SSL_Pinned

A different cert needs to be substituted when testing on a device
since the SG will not be on 'localhost', and this cert will have a
different digest of course. So compute the digest at runtime.
Fixes #882

# Conflicts:
#	Unit-Tests/ReplicatorInternal_Tests.m

Unit-Tests/ReplicatorInternal_Tests.m

@@ -292,7 +292,7 @@ - (void) test_08_Puller_SSL_Pinned {
                                       userInfo: nil]);
 
     Log(@"Now replicating with correct pinned cert...");
-    digest = @"29adb68f518347bfa2770723642414f89a4d352a";
+    digest = CBLHexSHA1Digest([self contentsOfTestFile: @"SelfSigned.cer"]);
     id lastSeq = replic8Continuous(db, remoteURL, NO, nil,
                                    @{kCBLReplicatorOption_PinnedCert: digest},
                                    nil);