Has the case for Switzerland's strong privacy laws been established and more importantly, has it been tested?
As for it being tested, I can assure you that it's taken very seriously. One ruling that demonstrates that is [1], in which Switzerland's highest court ruled that an individual's right to privacy has higher precedence than a copyright-owner's right to police copyright infringement.
There's also a constitutional right to privacy [2], though the Swiss constitution is a little different to the American one.
One notable and enormous hole in Switzerland's record however is the BÜPF [3], which, as I understand it, requires ISPs to log DNS requests, among other things. That shouldn't be relevant here though, so long as Quad9 doesn't become a telecommunications provider.
[0]: https://www.fedlex.admin.ch/eli/cc/1993/1945_1945_1945/en
[1]: https://www.swissinfo.ch/eng/privacy-triumphs-in-internet-pi...
It has many "ifs" but not a single "then". It's like a parent threatening a child not to do something in the hope that the child never responds with "or what?"
Authorities have issued several advisories against global actors but whether or not they decide to comply is purely based on political agenda.
In my opinion, the answer to your question (was the resilience of the Swiss data protection law tested) is no, simply because it can't be tested.