[test] Remove PowerMockTestCase

It is causing weird errors similar to [this one](robolectric/robolectric#2208):

```
Cannot convert type org.apache.tools.ant.Project to type org.apache.tools.ant.Project
```

And it doesn’t seem to serve a real purpose—at least not any longer.

Author: Clément Le Provost

algoliasearch/src/test/java/com/algolia/search/saas/BrowseIteratorTest.java

@@ -38,7 +38,7 @@
 
 import static junit.framework.Assert.fail;
 
-public class BrowseIteratorTest extends PowerMockTestCase {
+public class BrowseIteratorTest extends RobolectricTestCase {
     Client client;
     Index index;
     String indexName;

algoliasearch/src/test/java/com/algolia/search/saas/ClientTest.java

@@ -39,7 +39,7 @@
  */
 
 @SuppressLint("DefaultLocale") //We use format for logging errors, locale issues are irrelevant
-public class ClientTest extends PowerMockTestCase {
+public class ClientTest extends RobolectricTestCase {
     Client client;
 
     @Override

algoliasearch/src/test/java/com/algolia/search/saas/IndexTest.java

@@ -62,7 +62,7 @@
  */
 
 @SuppressLint("DefaultLocale") //We use format for logging errors, locale issues are irrelevant
-public class IndexTest extends PowerMockTestCase {
+public class IndexTest extends RobolectricTestCase {
     Client client;
     Index index;
     String indexName;