move the test helper utils from test/ to main/

So we can use the test helpers utils in other modules.

@bryanduxbury 
CC: @square/cascading-contributors 

Pinged @da3mon about this (he added the test a few days ago), the FlowBuilderTest.java doesn't run and gives a cannot resolve method hasItem on this line:
```java
    assertThat(builder.getEmittedClasses(), hasItem(Tests.Left.class));
```