Refactor and strengthen sdk_test.go.

This change:

- Removes usage of the EXPERIMENTAL_JAVA_LANGUAGE_LEVEL_9=false option
  from tests, since it is going to be removed. The java_version
  property is used instead.

- Commons up some of the assertions between the tests for language
  levels 8 and 9 in sdk_test.go.

- In commoning the code up, some additional assertions are made in the
  language level 9 cases, strengthening the tests.

Test: m nothing
Bug: 115604102

Change-Id: I693c5d299b5592b851c44dde4434d92a931f15cd