The Nazi policy towards the Czechs changed over time and ultimately invalidated that logic of racial hierarchy essential to Nazi ideology. As you noted, when Germany first occupied the Czech lands, the assumption was that they would shift through and find those of German blood an expel the rest. The Czech lands, and especially Bohemia and its capital Prague, had been heavily influenced by German culture prior to occupation. Prague had only gained its independence from the Austrian empire two decades prior to German occupation, and the city had been ruled by German speakers for centuries. Moreover, Prague's proximity to Germany itself (Prague is only a 4 hour drive to Berlin today) as well as the large German minority that made up the Sudeten region between Prague and Germany, ensured that a strong mix of German language and culture could be found in Prague. Thus when the Nazis sought to differentiate between Czechs that could be Germanized and the Czechs that would need to be expelled, they found a population that could wear a German hat in one context and a Czech hat in another--ethnicity was not as distinct as Nazi ideology necessitated and many "Czechs" became "Germans" during the occupation, only to become "Czechs" again after the war.

The ambiguity of ethnicity was not the only problem that faced the German plan for the expulsion of undesirables from Prague. As the Second World War progressed, the Germans found themselves in dire need of Czech industry (all industry that they controlled) to produce at a highly productive rate. Thus the German occupiers found that they needed to work with the Czechs on a scale unimagined a few years before. The occupiers dropped the idea of expelling the Czechs for the duration of the war, though it is not clear what fate the Czechs would have faced had the German won the war, however.

This does not mean there were no problems between the German occupiers and the Czechs. After the assassination of the governor of the Protectorate of Bohemia and Moravia Reinhard Heydrich (an operation led by the British but carried out by Czechoslovak agents), the Germans retaliated by razing a Czech town that had harbored the agents prior to the assassination. Nevertheless, the occupation did not proceed as the Germans had expected, as they found themselves confused as to who could be classified as a "German" and in need of the assistance of those "Czechs" they had originally intended to expel. The situation was far more complicated than this, however, for while the German occupation revealed the fluidity of ethnic identity, it also engendered a Czech nationalism that would have dire consequences for the Germans of the Sudeten region after the war.

The most comprehensive analysis of this topic is undoubtedly Chad Bryant's Prague in Black: Nazi Rule and Czech Nationalism, Harvard University Press, 2009.

P.S. -- I know this does not answer your question directly (and really I don't know anything about this test you are talking about), but you can see that the Germans had reasons to make peace with the Czechs that they did not have with other Slavic peoples (i.e., geography, industry, and shared culture).