test: Improve functional test debuggability and efficiency

- Read TEST_TAG/TEST_FILTER env vars from cmake/RunTests.cmake. Setting these
  environment variables will pass --tags/--filter to busted, which can used to
  filter which tests are executed.
- Remove calls to nvim msgpack-rpc API outside tests. This removes the
  requirement of having a static `clear` call in test/functional/helpers.lua
- Use the new busted command-line option "--lazy" to ensure the setup/teardown
  hooks are only executed when a suite runs at least one test.

Now its possible to run/debug a single test like this:

```sh
TEST_FILTER='some test string' make test
```

Which will only run tests containing "some test string" in the title.
Another option is:

```sh
TEST_TAG=some-tag make test
```

After putting #some-tag into the test title. This also improves debugging
experience because there will be no unnecessary gdbserver instances whe GDB=1 is
passed.

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