SecurityPkg: SecureBootVariableLib: Added unit tests

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=3911

This change added unit test and enabled it from pipeline for the updated
SecureBootVariableLib.

The unit test covers all implemented interfaces and certain corner cases.

Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Jian J Wang <jian.j.wang@intel.com>
Cc: Min Xu <min.m.xu@intel.com>

Signed-off-by: Kun Qin <kun.qin@microsoft.com>
Reviewed-by: Jiewen Yao <Jiewen.yao@intel.com>
Acked-by: Michael Kubacki <michael.kubacki@microsoft.com>

SecurityPkg/Library/SecureBootVariableLib/UnitTest/MockPlatformPKProtectionLib.c

@@ -0,0 +1,36 @@
+/** @file
+  Provides a mocked interface for configuring PK related variable protection.
+
+  Copyright (c) Microsoft Corporation.
+  SPDX-License-Identifier: BSD-2-Clause-Patent
+**/
+#include <stdio.h>
+#include <string.h>
+#include <stdarg.h>
+#include <stddef.h>
+#include <setjmp.h>
+#include <cmocka.h>
+
+#include <Uefi.h>
+
+/**
+  Disable any applicable protection against variable 'PK'. The implementation
+  of this interface is platform specific, depending on the protection techniques
+  used per platform.
+
+  Note: It is the platform's responsibility to conduct cautious operation after
+        disabling this protection.
+
+  @retval     EFI_SUCCESS             State has been successfully updated.
+  @retval     Others                  Error returned from implementation specific
+                                      underying APIs.
+
+**/
+EFI_STATUS
+EFIAPI
+DisablePKProtection (
+  VOID
+  )
+{
+  return (EFI_STATUS)mock ();
+}