Update contribution guide to not use TestGenerator (#92)

* Update contribution guide to not use TestGenerator.
* Update contribution guide to use compilerTestEnabled property.

CONTRIBUTING.md

@@ -57,12 +57,9 @@ it should be extending [AbstractTestProcessor](compiler-plugin/src/main/kotlin/c
         * Then follows virtual files section till the end of test file.
         * You can use ```// FILE: <file name>``` to create files that will be available at run time of the test.
             * E.g. ```// FILE: a.kt``` will result in a file named ```a.kt``` at run time.
-* Generate test using gradle.
-    * After you have finished writing your test file, you can generate the test case in TestSuite by running 
-    ```generateTests```gradle task. It can take a while. 
-    * After generating, make sure there is no other tests generated by checking git status, you should include only tests in 
-    [KotlinKSPTestGenerated](compiler-plugin/src/test/kotlin/com/google/devtools/ksp/test/KotlinKSPTestGenerated.java) in your PR.
+* Add new test to [test suite](compiler-plugin/src/test/java/com/google/devtools/ksp/test/KotlinKSPTestGenerated.java)
 * Run generated tests with ```:compiler-plugin:test``` gradle task.
+    * Make sure you run test with `-PcompilerTestEnabled` as Gradle argument. Alternatively, you can also set `compilerTestEnabled=false` in `gradle.properties` in project root.
     * This will execute all tests in KSP test suite. To run your test only, specify the test name with 
     ```--tests "com.google.devtools.ksp.test.KotlinKSPTestGenerated.<name of your generated test>"```
     * Make sure your change is not breaking any existing test as well :).