qmake generates broken Makefile

I'm not sure what's going on or what triggers the issue, but I think it's a qmake bug. The problem is easily reproducible on Windows and Linux (didn't try macOS):
1. download the attached archive and unpack it;
2. navigate to "tests";
3. open tests.pro in Qt Creator;
4. disable shadow build and hit menu - Build - Run qmake;
5. observe the error in Compile output.

Something about the structure of my project must be triggering it; I'll be grateful if someone can suggest a workaround.

Meanwhile, generating a VS project works fine (qmake -tp vc -r), the resulting projects is perfectly working.