[libcalamaresui] Fix up test for logfile

- this test would fail if the logfile already exists for
  any reason (including "I just ran the test")
- remove the file before expecting an empty logfile
- improve messages; a missing logfile is not a "things cannot
  work" situation, it's a warning

src/libcalamaresui/utils/Paste.cpp

@@ -28,10 +28,11 @@ using namespace CalamaresUtils::Units;
 STATICTEST QByteArray
 logFileContents()
 {
-    QFile pasteSourceFile( Logger::logFile() );
+    const QString name = Logger::logFile();
+    QFile pasteSourceFile( name );
     if ( !pasteSourceFile.open( QIODevice::ReadOnly | QIODevice::Text ) )
     {
-        cError() << "Could not open log file";
+        cWarning() << "Could not open log file" << name;
         return QByteArray();
     }
     QFileInfo fi( pasteSourceFile );

src/libcalamaresui/utils/TestPaste.cpp

@@ -35,15 +35,16 @@ private Q_SLOTS:
 void
 TestPaste::testGetLogFile()
 {
+    QFile::remove( Logger::logFile() );
     // This test assumes nothing **else** has set up logging yet
-    QByteArray b = logFileContents();
+    QByteArray contentsOfLogfileBefore = logFileContents();
-    QVERIFY( b.isEmpty() );
+    QVERIFY( contentsOfLogfileBefore.isEmpty() );
 
     Logger::setupLogLevel( Logger::LOGDEBUG );
     Logger::setupLogfile();
 
-    b = logFileContents();
+    QByteArray contentsOfLogfileAfterSetup = logFileContents();
-    QVERIFY( !b.isEmpty() );
+    QVERIFY( !contentsOfLogfileAfterSetup.isEmpty() );
 }
 
 void