CppEditor: Proliferate FilePath use

Remove SnapShot::{find,contains}(QString) overloads and fix fallout.

Change-Id: I50c415826939e09ac111f33def611c7478fa5d97
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Christian Stenger <christian.stenger@qt.io>
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/coreplugin/testdatadir.h

@@ -15,6 +15,8 @@
             : TestDataDir(QLatin1String(SRCDIR "/" PATH) + testDataDirectory) {} \
     };
 
+namespace Utils { class FilePath; }
+
 namespace Core {
 namespace Tests {
 
@@ -24,6 +26,7 @@ public:
     TestDataDir(const QString &directory);
 
     QString file(const QString &fileName) const;
+    Utils::FilePath filePath(const QString &fileName) const;
     QString directory(const QString &subdir = QString(), bool clean = true) const;
 
     QString path() const;