Enable Proguard testing directives via environment flag

Bug: 142108662
Change-Id: I9784b55bed9896fe5a625e0b4093ea6739dcfeb3
diff --git a/src/main/java/com/android/tools/r8/R8Command.java b/src/main/java/com/android/tools/r8/R8Command.java
index ae68adb..f4b3763 100644
--- a/src/main/java/com/android/tools/r8/R8Command.java
+++ b/src/main/java/com/android/tools/r8/R8Command.java
@@ -101,7 +101,8 @@
     private final List<FeatureSplit> featureSplits = new ArrayList<>();
 
     private boolean allowPartiallyImplementedProguardOptions = false;
-    private boolean allowTestProguardOptions = false;
+    private boolean allowTestProguardOptions =
+        System.getProperty("com.android.tools.r8.allowTestProguardOptions") != null;
 
     // TODO(zerny): Consider refactoring CompatProguardCommandBuilder to avoid subclassing.
     Builder() {