[Microsoft.Build.Engine] Fix race condition in TestTasks.dll compilation and embeddin...

[Microsoft.Build.Engine] Fix race condition in TestTasks.dll compilation and embedding (#11275)

Since we've started embedding test resources inside of the test assembly
we've sometimes seen a strange error on Jenkins during the test run:

```
Msg: Error initializing task StringTestTask: Could not load file or assembly 'Microsoft.Build.Utilities.Core, Version=14.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' or one of its dependencies.
Msg: Error initializing task StringTestTask: System.TypeLoadException: Could not load file or assembly 'Microsoft.Build.Utilities.Core, Version=14.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' or one of its dependencies.
  at (wrapper managed-to-native) System.RuntimeType.GetConstructors_native(System.RuntimeType,System.Reflection.BindingFlags)
...
```

However this was from an xbuild_12 run so how come it tried to load xbuild 14 stuff?

The reason is there's a race condition between when TestTasks.dll gets
built and when it's embedded inside of the test assembly.

If we're building the xbuild_12 and xbuild_14 profile at the same time
we could end up embedding the xbuild_14 version of TestTasks.dll instead
of the correct one since we're copying it to the same file name.

The fix is to remove the unnecessary copying logic since we can just tell
the compiler to embed the file under the correct name.