Fix compilation error with gcc 7.4.1, ambiguous overload

Summary:
test_lldbformatters.cpp: In member function ‘void LldbFormattersTest::testQSetString()’:
test_lldbformatters.cpp:837:62: error: call of overloaded ‘verifyVariable(int, QString, QString, <brace-enclosed initializer list>, [...])’ is ambiguous
                         __FILE__, __LINE__, true, false, true)) {
                                                              ^
test_lldbformatters.cpp:231:6: note: candidate: bool LldbFormattersTest::verifyVariable(int, const QString&, const QString&, QList<QPair<QString, QString> >, [...])
test_lldbformatters.cpp:208:6: note: candidate: bool LldbFormattersTest::verifyVariable(int, const QString&, const QString&, QStringList, [...])

Test Plan: Builds

Reviewers: kfunk, kossebau

Subscribers: kdevelop-devel

Tags: #kdevelop

Differential Revision: https://phabricator.kde.org/D25639