test/support: consolidate requireNoBfpWire (7 copies) + ASSERT_EXITS_WITH_MESSAGE DeathTest variant

Follow-up 2 from PR #413 (BFP epic #410).

PR #413 added storage-dtype guards to every outside-funnel layer that raw-reads wires (Relu, Dropout, Flatten, Softmax backward, all three pool backwards). The guard tests replicate a local `requireNoBfpWire` helper **7 times**; consolidate it into the shared test support library (`test/unit/support/`, next to the `BorrowedLayer.h` builders).

While at it: `DeathTest.h` gains an `ASSERT_EXITS_WITH_MESSAGE` variant so guard tests can assert on the guided message text, not just the exit code — the guided messages ("BFP epic PRn") are part of the contract and currently unpinned.

Start in test/unit/support/ beside the BorrowedLayer.h builders and locate the seven guard-test copies of requireNoBfpWire. Read DeathTest.h and the existing guard tests to understand the exit assertions. Done means one shared helper is used and the guard tests assert the guided “BFP epic PRn” messages through the new variant.

Written by the indexing model from the issue text.