CI : Test IECoreUSD on Windows

I thought #1204 had done this already, but it seems to have done all the hard work, but not actually thrown the switch.

Author: johnhaddon

.github/workflows/main.yml

@@ -90,7 +90,7 @@ jobs:
             buildType: RELWITHDEBINFO
             options: .github/workflows/main/options.windows
             dependenciesURL: https://github.com/hypothetical-inc/gafferDependencies/releases/download/3.1.1/gafferDependencies-3.1.1-Python3-windows.zip
-            tests: testCore testCorePython testScene testImage testAlembic testVDB
+            tests: testCore testCorePython testScene testImage testAlembic testUSD testVDB
             publish: false
 
     runs-on: ${{ matrix.os }}