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/autotest/autotest.qbs

@@ -1,5 +1,3 @@
-import qbs
-
 QtcPlugin {
     name: "AutoTest"
 
@@ -18,16 +16,8 @@ QtcPlugin {
         "QmakeProjectManager"
     ]
 
-    Depends {
-        name: "QtSupport"
-        condition: qtc.testsEnabled
-    }
-
-    Depends {
-        name: "Qt.testlib"
-        condition: qtc.testsEnabled
-    }
-
+    Depends { name: "QtSupport"; condition: qtc.withPluginTests }
+    Depends { name: "Qt.testlib"; condition: qtc.withPluginTests }
     Depends { name: "Qt.widgets" }
 
     files: [