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.

cmake/RunTests.cmake

@@ -17,9 +17,17 @@ if(BUSTED_OUTPUT_TYPE STREQUAL junit)
   set(EXTRA_ARGS OUTPUT_FILE ${BUILD_DIR}/${TEST_TYPE}test-junit.xml)
 endif()
 
+if(DEFINED ENV{TEST_TAG})
+  set(TEST_TAG "--tags=$ENV{TEST_TAG}")
+endif()
+
+if(DEFINED ENV{TEST_FILTER})
+  set(TEST_TAG "--filter=$ENV{TEST_FILTER}")
+endif()
+
 execute_process(
-  COMMAND ${BUSTED_PRG} -v -o ${BUSTED_OUTPUT_TYPE}
-    --helper=${TEST_DIR}/${TEST_TYPE}/preload.lua
+  COMMAND ${BUSTED_PRG} ${TEST_TAG} ${TEST_FILTER} -v -o ${BUSTED_OUTPUT_TYPE}
+    --lazy --helper=${TEST_DIR}/${TEST_TYPE}/preload.lua
     --lpath=${BUILD_DIR}/?.lua ${TEST_PATH}
   WORKING_DIRECTORY ${WORKING_DIR}
   ERROR_VARIABLE err

test/functional/autocmd/tabnew_spec.lua

@@ -4,8 +4,8 @@ local clear, nvim, buffer, curbuf, curwin, eq, neq, ok =
   helpers.eq, helpers.neq, helpers.ok
 
 describe('TabNew', function()
+    setup(clear)
     describe('au TabNew', function()
-        clear()
         describe('with * as <afile>', function()
             it('matches when opening any new tab', function()
                 nvim('command', 'au! TabNew * echom "tabnew:".tabpagenr().":".bufnr("")')

test/functional/helpers.lua

@@ -276,8 +276,6 @@ local function expect(contents)
   return eq(dedent(contents), curbuf_contents())
 end
 
-clear()
-
 return {
   clear = clear,
   spawn = spawn,

test/functional/job/job_spec.lua

@@ -6,10 +6,14 @@ local clear, nvim, eq, neq, ok, expect, eval, next_message, run, stop, session
   helpers.stop, helpers.session
 local nvim_dir, insert = helpers.nvim_dir, helpers.insert
 
-local channel = nvim('get_api_info')[1]
 
 describe('jobs', function()
-  before_each(clear)
+  local channel
+
+  before_each(function()
+    clear()
+    channel = nvim('get_api_info')[1]
+  end)
 
   -- Creates the string to make an autocmd to notify us.
   local notify_str = function(expr1, expr2)