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/mesonprojectmanager/mesonprojectmanager.qbs

@@ -95,7 +95,6 @@ Project {
 
     QtcAutotest {
         name: "mesonwrapper"
-        condition: project.withAutotests
 
         Depends { name: "Core" }
         Depends { name: "Utils" }
@@ -116,7 +115,6 @@ Project {
 
     QtcAutotest {
         name: "mesoninfoparser"
-        condition: project.withAutotests
 
         Depends { name: "Core" }
         Depends { name: "Utils" }
@@ -138,7 +136,6 @@ Project {
 
     QtcAutotest {
         name: "ninjaparser"
-        condition: project.withAutotests
 
         Depends { name: "Core" }
         Depends { name: "ProjectExplorer" }
@@ -156,7 +153,6 @@ Project {
 
     QtcAutotest {
         name: "mesonparser"
-        condition: project.withAutotests
 
         Depends { name: "Core" }
         Depends { name: "ProjectExplorer" }