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.

1 files changed, 10 insertions, 2 deletions

diff --git a/cmake/RunTests.cmake b/cmake/RunTests.cmake
index 6262cbc383..0858ea24ac 100644
--- a/cmake/RunTests.cmake
+++ b/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