Allow namespace config to be tested properly

Previously, the NewTextContext created a context that always called
NewNameResolver with a export filter that always returned true.

This change fixes that by:
1. Changing NewNameResolver to take a Config parameter instead of a
   filter parameter and pushing the code to create the filter from the
   Config from newNameResolver() in cmd/soong_build/main.go into the
   NewNameResolver function.

2. Extracting a newTestContextForFixture that does not create a
   NameResolver or set it on the context. That avoids creating a
   NameResolver before the test has prepared the config.

3. Modify the fixture to create and set the NameResolver in the Context
   after the config has been prepared by the test.

4. Added test to verify that it all works.

Bug: 234825639
Test: m nothing
Change-Id: Ie4b13f18093dc01a0ab65a9ecfd143585d843b55