Revert "Run tests with Gradle instead of IDEA's test runner"

This reverts commit 7415ae6ec9.

The problems that caused me to write this commit in the first place have
been fixed. Using IDEA's test runner directly is nicer - e.g. for code
coverage integration.

.idea/runConfigurations/dev_openrs2.xml

@@ -0,0 +1,16 @@
+<component name="ProjectRunConfigurationManager">
+  <configuration default="false" name="dev.openrs2" type="JUnit" factoryName="JUnit">
+    <option name="PACKAGE_NAME" value="dev.openrs2" />
+    <option name="MAIN_CLASS_NAME" value="" />
+    <option name="METHOD_NAME" value="" />
+    <option name="TEST_OBJECT" value="package" />
+    <option name="VM_PARAMETERS" value="-ea -Dio.netty.leakDetection.level=PARANOID" />
+    <option name="PARAMETERS" value="" />
+    <option name="TEST_SEARCH_SCOPE">
+      <value defaultName="wholeProject" />
+    </option>
+    <method v="2">
+      <option name="Make" enabled="true" />
+    </method>
+  </configuration>
+</component>

.idea/runConfigurations/test.xml

@@ -1,22 +0,0 @@
-<component name="ProjectRunConfigurationManager">
-  <configuration default="false" name="test" type="GradleRunConfiguration" factoryName="Gradle">
-    <ExternalSystemSettings>
-      <option name="executionName" />
-      <option name="externalProjectPath" value="$PROJECT_DIR$" />
-      <option name="externalSystemIdString" value="GRADLE" />
-      <option name="scriptParameters" value="" />
-      <option name="taskDescriptions">
-        <list />
-      </option>
-      <option name="taskNames">
-        <list>
-          <option value="cleanTest" />
-          <option value="test" />
-        </list>
-      </option>
-      <option name="vmOptions" value="" />
-    </ExternalSystemSettings>
-    <GradleScriptDebugEnabled>true</GradleScriptDebugEnabled>
-    <method v="2" />
-  </configuration>
-</component>