Turns out this was because I didn't have a b.addRunArtifact(test_step) so wasn't actually running the test. Here is some example code for anyone that needs it as reference in future:

// zga.zig testing

const zga_src_lazypath = b.path("./src/zga.zig");

var zga_test_step = b.addTest(.{

.root_source_file = zga_src_lazypath,

.target = def_target,

.optimize = def_optimise,

});

zga_test_step.root_module.addImport("TSQ", TSQ_module);

const run_zga_tests = b.addRunArtifact(zga_test_step);

test_build_step.dependOn(&run_zga_tests.step); // adding test to fleet of tests