qbs build: Clean up testing infrastructure

- Rename ambiguously named "testsEnabled" property to "withPluginTests"
  and fix the wrong uses.
- Move AutotestRunner out of main project file and into tests.qbs.
- Move "withAutotests" property into qtc module. We cannot remove it
  from the main project file just yet, because some manual tests do not
  make use of QtcManualTest at the moment.
- Do not remove the autotests from the project tree if autotests are
  disabled. Instead, just disable the products.

Change-Id: I74a4cc717d9adbe53d37536b2565111e1617115b
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/clangcodemodel/clangcodemodel.qbs

@@ -10,7 +10,7 @@ QtcPlugin {
     Depends { name: "CppEditor" }
     Depends { name: "LanguageClient" }
     Depends { name: "ProjectExplorer" }
-    Depends { name: "QtSupport"; condition: qtc.testsEnabled }
+    Depends { name: "QtSupport"; condition: qtc.withPluginTests }
     Depends { name: "TextEditor" }
     Depends { name: "Utils" }