libp2p v2.0.0's `crypto.newRng()` returns the new `Rng` wrapper type,
which can't be assigned to a `ref HmacDrbgContext` field. The two test
helper modules (tests/testlib/common.nim and tests/test_helpers.nim)
both fall in this pattern; rest of the codebase migrated in the prior
sweep but these two test-only files were missed because they don't
compile during the wakunode2 / chat2mix builds — only when the test
target is built (which CI does, locally we didn't until this PR).
Same fix pattern as the other newRng → HmacDrbgContext.new() migrations.
