Gate dummy unused H265 cros-codecs unit test

This change gates dummy backend implementations and related failing
cros-codecs unit tests, which are not working and not in use for the
time being, with`#[cfg(feature = "dummy")]`.

Test: presubmit build
Test: m with aosp_x86_64-trunk_staging-userdebug target
Bug: 442013649
Bug: 438731084
Flag: EXEMPT test only
Change-Id: I8462393c47b57c5cf40cee033f14ac9d9822d5e9
diff --git a/src/decoder/stateless/h265/vaapi.rs b/src/decoder/stateless/h265/vaapi.rs
index 846945d..3caa870 100644
--- a/src/decoder/stateless/h265/vaapi.rs
+++ b/src/decoder/stateless/h265/vaapi.rs
@@ -888,6 +888,7 @@
     }
 }
 
+#[cfg(feature = "dummy")]
 #[cfg(test)]
 mod tests {
     use libva::Display;