Basic dockerised test suite.

This introduces a test suite for libssh2. It runs OpenSSH in a Docker
container because that works well on Windows (via docker-machine) as
well as Linux. Presumably it works on Mac too with docker-machine, but
I've not tested that.

Because the test suite is docker-machine aware, you can also run it
against a cloud provider, for more realistic network testing, by setting
your cloud provider as your active docker machine. The Appveyor CI setup
in this commit does that because Appveyor doesn't support docker
locally.

tests/test_hostkey.c

@@ -0,0 +1,54 @@
+#include "session_fixture.h"
+
+#include <libssh2.h>
+
+#include <stdio.h>
+
+const char *EXPECTED_HOSTKEY =
+    "AAAAB3NzaC1yc2EAAAABIwAAAQEArrr/JuJmaZligyfS8vcNur+mWR2ddDQtVdhHzdKU"
+    "UoR6/Om6cvxpe61H1YZO1xCpLUBXmkki4HoNtYOpPB2W4V+8U4BDeVBD5crypEOE1+7B"
+    "Am99fnEDxYIOZq2/jTP0yQmzCpWYS3COyFmkOL7sfX1wQMeW5zQT2WKcxC6FSWbhDqrB"
+    "eNEGi687hJJoJ7YXgY/IdiYW5NcOuqRSWljjGS3dAJsHHWk4nJbhjEDXbPaeduMAwQU9"
+    "i6ELfP3r+q6wdu0P4jWaoo3De1aYxnToV/ldXykpipON4NPamsb6Ph2qlJQKypq7J4iQ"
+    "gkIIbCU1A31+4ExvcIVoxLQw/aTSbw==";
+
+int test(LIBSSH2_SESSION *session)
+{
+    int rc;
+    size_t len;
+    int type;
+    unsigned int expected_len = 0;
+    char *expected_hostkey = NULL;
+
+    const char *hostkey = libssh2_session_hostkey(session, &len, &type);
+    if (hostkey == NULL) {
+        print_last_session_error("libssh2_session_hostkey");
+        return 1;
+    }
+
+    if (type != LIBSSH2_HOSTKEY_TYPE_RSA) {
+        /* Hostkey configured in docker container is RSA */
+        fprintf(stderr, "Wrong type of hostkey\n");
+        return 1;
+    }
+
+    rc = libssh2_base64_decode(session, &expected_hostkey, &expected_len,
+                               EXPECTED_HOSTKEY, strlen(EXPECTED_HOSTKEY));
+    if (rc != 0) {
+        print_last_session_error("libssh2_base64_decode");
+        return 1;
+    }
+
+    if (len != expected_len) {
+        fprintf(stderr, "Hostkey does not have the expected length %ld != %d\n",
+                len, expected_len);
+        return 1;
+    }
+
+    if (memcmp(hostkey, expected_hostkey, len) != 0) {
+        fprintf(stderr, "Hostkeys do not match\n");
+        return 1;
+    }
+
+    return 0;
+}