Remove bats as a requirements for testing

[underwoo]

We have had issues with bats in the past (leaving long-running processes when a test fails), and while getting native TAP output is nice, debugging failing tests was challenging -- especially when the script has multiple tests.

Another issue users who build/instal FRE-NCtools may not know that test will only run if bats is installed, and in the user's PATH -- as configure.ac doesn't fail if bats is not found. We should rewrite the tests not to use bats.