Enable -strict for user tests

[niloc132]

Follow-up to #10037

With the -strict flag supported for tests, we can use this flag to ensure our source paths are complete and included in module files.

Presently they are not - at least com.google.gwt.testing.TestUtils is referenced by classes that appear in other modules. com.google.gwt.storage.client.MapInterfaceTest for example references TestUtils, but appears in the com.google.gwt.storage.client package, which is part of Storage.gwt.xml.

• This fails GwtServletBaseTest,
• which inherits JUnit (note that it shouldn't JUnit is inherited automatically)
• which inherits User
• which inherits Storage (not storage test)
• which includes tests in com.google.gwt.storage.client

Since this fails through the JUnit module, which is automatically inherited by every GWTTestCase module, many tests in gwt-user's own tests would fail in this way. There are likely other issues like this one, but this is the most obvious as it fails most every test.

On the plus side, this validates that -strict works for tests as expected.