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.inf

@@ -0,0 +1,25 @@
+## @file
+#  Mock implementation of the UEFI Runtime Services Table Library.
+#
+#  Copyright (c) 2020, Intel Corporation. All rights reserved.<BR>
+#  SPDX-License-Identifier: BSD-2-Clause-Patent
+#
+##
+
+[Defines]
+  INF_VERSION                    = 0x00010005
+  BASE_NAME                      = MockUefiRuntimeServicesTableLib
+  FILE_GUID                      = 84CE0021-ABEE-403C-9A1B-763CCF2D40F1
+  MODULE_TYPE                    = UEFI_DRIVER
+  VERSION_STRING                 = 1.0
+  LIBRARY_CLASS                  = UefiRuntimeServicesTableLib|HOST_APPLICATION
+
+#
+#  VALID_ARCHITECTURES           = IA32 X64 EBC
+#
+
+[Sources]
+  MockUefiRuntimeServicesTableLib.c
+
+[Packages]
+  MdePkg/MdePkg.dec