MdePkg: Add Google Mock Library for PostCodeLib

[TsunFeng]

Description

MdePkg: Add Google Mock Library for PostCodeLib

• [ ] Breaking change?
  • Breaking change - Does this PR cause a break in build or boot behavior?
  • Examples: Does it add a new library class or move a module to a different repo.
• [ ] Impacts security?
  • Security - Does this PR have a direct security impact?
  • Examples: Crypto algorithm change or buffer overflow fix.
• [ ] Includes tests?
  • Tests - Does this PR include any explicit test code?
  • Examples: Unit tests or integration tests.

How This Was Tested

Unit tests component can call these mock functions success

Integration Instructions

N/A

[mergify[bot]]

PR can not be merged due to conflict. Please rebase and resubmit

[lgao4]

Can you remove merge commit?

[mdkinney]

The commit message is not complete: Please see https://github.com/tianocore/tianocore.github.io/wiki/Commit-Message-Format