Simplify PostBuild.mak for easier maintenance

[jng]

Right now PostBuild.mak copies *individual* data files from sub-directories under UnitTest to the target test data directory for the server test suite.

It then duplicates these copy directives for every debug/release and msvc compiler version. As a result, this makes maintaining this file (eg. Adding new data for a new server test case) an absolute PITA!

PostBuild.mak is only used for the Windows build as Linux does its own post-build test data setup, yet I rarely recall ever having to update the Linux post-build whenever I added new test cases, so the Linux post-build must be doing something right and PostBuild.mak should follow their example

[jng]

In 10022:

Simplify the test data preparation step by having a single CopyTestDataFiles target that does wildcard-based xcopy of the required source test data directories to its target destination. It's not a recursive GLOB like the Linux (CMake) build, but it is close enough in that new test files added in the future should no longer require modifications to PostBuild.mak. It should be caught by the new wildcard-based xcopy.

Fixes #2860