datatest: enable listing of tests
datatest: enable listing of tests

Teach test binaries built using `datatest-stable` to be able to list the
names of each test with the `--list` option. This matches the behavior
of the `--list` option with test binaries built using the default test
harness.

Closes: #3318
Approved by: sausagee

aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core,libra/libra,libra/libra,libra/libra,libra/libra,aptos-labs/aptos-core,libra/libra