tests: move/rename/cleanup vspec based tests (#860)

The main motivation is to being able to use normal/other pytest based
tests.

Therefore this moves everything from conftest into test_integration
itself.

Makefile

@@ -1,8 +1,8 @@
 test:
-	pytest test_*.py
+	pytest
 
 test_nvim:
-	VSPEC_VIM=nvim pytest test_*.py
+	VSPEC_VIM=nvim pytest
 
 test_coverage: export PYTEST_ADDOPTS:=--cov pythonx --cov test --cov-report=term-missing:skip-covered
 test_coverage: test_nvim