Fix `test-morley-client.bats` when bats-1.2.1 is available in most linux distros

Clarification and motivation

We'd like to generate sender and receiver keys in test-morley-client.bats only once before all test, but this is only possible if bats version if newer than 1.2.1.

Unfortunately, on some linux distros (e.g Ubuntu 20.04) the newest version of bats is 1.1.0 (for more context, see discussion in this MR), so in this case we have to generate them before each test, which slows down their execution

When 1.2.1 version of bats will be available in LTS of most popular linux distros, we need to remove this feature from test-morley-client.bats to speed up the local test execution.

Acceptance criteria

When bats-1.2.1 will be available in LTS of most popular linux distro, the TODO in test-morley-client.bats is resolved.