SF: Verify mDebugDisableHWC forces client composition

Add a composition test to verify that setting setting mDebugDisableHWC
to true causes client composition to be performed.

Test: atest libsurfaceflinger_unittest
Bug: None
Change-Id: Iad5e8fbe4d77ba61460e03c5dfa9c0686e11fcc5
diff --git a/services/surfaceflinger/tests/unittests/TestableSurfaceFlinger.h b/services/surfaceflinger/tests/unittests/TestableSurfaceFlinger.h
index b77f82a..926b8ac 100644
--- a/services/surfaceflinger/tests/unittests/TestableSurfaceFlinger.h
+++ b/services/surfaceflinger/tests/unittests/TestableSurfaceFlinger.h
@@ -356,6 +356,7 @@
     auto& mutableTransactionFlags() { return mFlinger->mTransactionFlags; }
     auto& mutableUseHwcVirtualDisplays() { return mFlinger->mUseHwcVirtualDisplays; }
     auto& mutablePowerAdvisor() { return mFlinger->mPowerAdvisor; }
+    auto& mutableDebugDisableHWC() { return mFlinger->mDebugDisableHWC; }
 
     auto& mutableComposerSequenceId() { return mFlinger->getBE().mComposerSequenceId; }
     auto& mutableHwcDisplayData() { return getHwComposer().mDisplayData; }