Replace every httptest.NewRequest call with
httptest.NewRequestWithContext using the test's t.Context(). Thread
t *testing.T through the createSetupFormRequest and
createLoginFormRequest helpers so they can supply a context.
make lint under golangci-lint 2.12.2 drops from 94 findings to 47
(remaining: 23 gosec, 24 goconst, tracked in #176/#177/#178). make
test and make fmt-check pass unchanged.
Closes#175