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/MockUefiRuntimeServicesTableLib.c

@@ -0,0 +1,13 @@
+/** @file
+  Mock implementation of the UEFI Runtime Services Table Library.
+
+  Copyright (C) Microsoft Corporation.
+  SPDX-License-Identifier: BSD-2-Clause-Patent
+
+**/
+
+#include <Uefi.h>
+
+extern EFI_RUNTIME_SERVICES  gMockRuntime;
+
+EFI_RUNTIME_SERVICES  *gRT = &gMockRuntime;