The test cases
SmartImportTests.testConfigurationFullBuildAfterImportedProjects and
ImportExistingProjectsWizardTest.test20FullBuildAfterImportedProjectsZipFile
(as well as subsequent tests cases in ImportExistingProjectsWizardTest)
randomly fail because the test builder is not triggered as expected.
Potential reasons for this are race conditions (because the test builder
resets its counters on its own, even though its consumers also do that)
and missing test isolation (auto-building not being enabled as
required).

This change addresses the potential reasons:
* Remove the unnecessary and potentially conflicting reset of counters
within the TestBuilder
* Ensure enabled auto-build via test setup (and properly restore default
settings afterwards)

May fix https://github.com/eclipse-platform/eclipse.platform.ui/issues/811
May fix https://github.com/eclipse-platform/eclipse.platform.ui/issues/1827