[Chore] Gen keys once in test-morley-client.bats

Description

Problem: scripts/test-morley-client.bats generates sender/receiver keys n+1 times, where n is the number of tests. Since key generation/ revelation takes a considerable amount of time, it would be better to only do this once

Solution: since bats v1.2.1, a setup_file hook exists that runs only once per testsuite. Use it for initial setup.

Related issue(s)

Initially brought up in #640 (comment 643038630)

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Related changes (conditional)

• Tests (see short guidelines)

  • If I added new functionality, I added tests covering it.
  • If I fixed a bug, I added a regression test to prevent the bug from silently reappearing again.

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N/A, this is a test script update

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