I don’t know how I got the idea, but now it works:
The issue was either the wrong entry point being loaded when having both _entry and _start (somehow Zig ignores my _entry function and uses _start, even though in the linker script I have clearly stated ENTRY(_entry)) or a symbol collision when using _start in either a separate startup program or everything.
My solution was to leave everything that will be needed when doing @import("root") in the src/test_main.zig file but move the test main function to src/main.zig. Then, I specified src/test_main.zig as the test runner (which didn’t contain the actual test runner code) and src/main.zig as the root source file. src/main.zig exports _start (and there it’ll check whether it’s in testing mode or not and conditionally invoke tmain or kmain).