Remove the use of InjectRule in UrlFetchUtilsTest

Random used to be a static variable which requires InjectRule to mock it in unit tests. It is now a singleton, which ensures that the same instance is called every time and Random.nextBytes() generates results that distribute uniformly between each call.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=217592767

javatests/google/registry/util/UrlFetchUtilsTest.java

@@ -52,10 +52,10 @@ public class UrlFetchUtilsTest {
   @Rule
   public final InjectRule inject = new InjectRule();
 
+  private final Random random = mock(Random.class);
+
   @Before
   public void setupRandomZeroes() {
-    Random random = mock(Random.class);
-    inject.setStaticField(UrlFetchUtils.class, "random", random);
     doAnswer(
             info -> {
               Arrays.fill((byte[]) info.getArguments()[0], (byte) 0);
@@ -69,7 +69,12 @@ public class UrlFetchUtilsTest {
   public void testSetPayloadMultipart() {
     HTTPRequest request = mock(HTTPRequest.class);
     setPayloadMultipart(
-        request, "lol", "cat", CSV_UTF_8, "The nice people at the store say hello. ヘ(◕。◕ヘ)");
+        request,
+        "lol",
+        "cat",
+        CSV_UTF_8,
+        "The nice people at the store say hello. ヘ(◕。◕ヘ)",
+        random);
     ArgumentCaptor<HTTPHeader> headerCaptor = ArgumentCaptor.forClass(HTTPHeader.class);
     verify(request, times(2)).addHeader(headerCaptor.capture());
     List<HTTPHeader> addedHeaders = headerCaptor.getAllValues();
@@ -97,7 +102,7 @@ public class UrlFetchUtilsTest {
     IllegalStateException thrown =
         assertThrows(
             IllegalStateException.class,
-            () -> setPayloadMultipart(request, "lol", "cat", CSV_UTF_8, payload));
+            () -> setPayloadMultipart(request, "lol", "cat", CSV_UTF_8, payload, random));
     assertThat(thrown)
         .hasMessageThat()
         .contains(