Improvements to bats-core integration

[jansorg]

• [x] Autocomplete common bats variables, $BATS_TEST_FILENAME and others
• [x] support bats keywords load
• [x] support bats keywords skip, run
• [ ] Support fd 3 to print inside of test functions
• [ ] Detect printing of code outside of @test functions, these must redirect to stderr
• [ ] intention to create a test function for an existing function (?)