The "new block chain json tests" were being skipped on Linux, but silently so
that CI didn't notice. These are a significant part of the Ethereum test suite.
See the missing output from `make test`, also visible in CI logs for Linux
targets (prior to this commit):
[OK] tests/fixtures/eth_tests/TransactionTests/ttGasPrice/TransactionWithGasPriceOverflow.json
[OK] tests/fixtures/eth_tests/TransactionTests/ttGasPrice/TransactionWithHighGasPrice.json
[Suite] new block chain json tests
<-- nothing here
[Suite] Fork ID tests
[OK] MainNet
[OK] RopstenNet
Commit 3d468a7 (`fixes path pointing to eth_tests`) renamed the JSON fixture
source directoryf in the witness-builder tests but not the regular blockchain
tests. As a result, searching for JSON test files yielded zero results. To
make this less likely in future, zero results is now an error.
Signed-off-by: Jamie Lokier <jamie@shareable.org>