CppEditor: Tweak test infrastructure

... so we can re-use the "follow symbol" test cases with clangd.

Call Creator like this (clangd needs to be version 12 or later):
$ QTC_CLANGD=<path to clangd> qtcreator -test
'CppEditor,*Follow*,*Switch*' -test 'ClangCodeModel,*dummy*'

During testing, some invalid code in the test cases was uncovered and
fixed.

Change-Id: I9dc650fdba2a27600e6a550420ee873f6fb31d23
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/cpptools/cpptoolstestcase.cpp

@@ -44,6 +44,7 @@
 #include <cplusplus/CppDocument.h>
 #include <utils/executeondestruction.h>
 #include <utils/fileutils.h>
+#include <utils/hostosinfo.h>
 #include <utils/temporarydirectory.h>
 
 #include <QtTest>
@@ -421,5 +422,15 @@ bool VerifyCleanCppModelManager::isClean(bool testOnlyForCleanedProjects)
 
 #undef RETURN_FALSE_IF_NOT
 
+int clangdIndexingTimeout()
+{
+    const QByteArray timeoutAsByteArray = qgetenv("QTC_CLANGD_INDEXING_TIMEOUT");
+    bool isConversionOk = false;
+    const int intervalAsInt = timeoutAsByteArray.toInt(&isConversionOk);
+    if (!isConversionOk)
+        return Utils::HostOsInfo::isWindowsHost() ? 20000 : 10000;
+    return intervalAsInt;
+}
+
 } // namespace Tests
 } // namespace CppTools