Enforce VoidMissingNullable in testonly code

The relaxation was useful to code NullMarked their production sources, but didn't bother with their test code. However, projects that want to check their entire codebase don't have great options as `-XepOpt:Nullness:Conservative` gates a lot of behavior behind it.

As we push further into null-marking code, annotating the types in test code will likely be beneficial anyway.

PiperOrigin-RevId: 707539601

core/src/main/java/com/google/errorprone/bugpatterns/nullness/VoidMissingNullable.java

@@ -92,10 +92,6 @@ public Description matchParameterizedType(
       return NO_MATCH;
     }
 
-    if (beingConservative && state.errorProneOptions().isTestOnlyTarget()) {
-      return NO_MATCH;
-    }
-
     if (beingConservative && !isInNullMarkedScope(state)) {
       return NO_MATCH;
     }
@@ -127,10 +123,6 @@ private static boolean isInNullMarkedScope(VisitorState state) {
 
   @Override
   public Description matchMethod(MethodTree tree, VisitorState state) {
-    if (beingConservative && state.errorProneOptions().isTestOnlyTarget()) {
-      return NO_MATCH;
-    }
-
     MethodSymbol sym = getSymbol(tree);
     if (!typeMatches(sym.getReturnType(), sym, state)) {
       return NO_MATCH;
@@ -143,10 +135,6 @@ public Description matchMethod(MethodTree tree, VisitorState state) {
 
   @Override
   public Description matchVariable(VariableTree tree, VisitorState state) {
-    if (beingConservative && state.errorProneOptions().isTestOnlyTarget()) {
-      return NO_MATCH;
-    }
-
     if (hasImplicitType(tree, state)) {
       /*
        * In the case of `var`, a declaration-annotation @Nullable would be valid. But a type-use