test: run tests sequentially

This seems to already be done in CI, so do it locally too. It helps with
ensuring that the compiler's filesystem cache stays consistent during
tests. Plus we don't have that many tests at the moment, so the increase
in test duration won't be that noticeable.

Makefile

@@ -40,7 +40,7 @@ test: $(PLENARY_DIR)
 		--headless \
 		--noplugin \
 		-u test/minimal_init.vim \
-		-c "PlenaryBustedDirectory $(TEST_DIR) { minimal_init = './test/minimal_init.vim' }"
+		-c "PlenaryBustedDirectory $(TEST_DIR) { minimal_init = './test/minimal_init.vim', sequential = true }"
 
 $(PLENARY_DIR):
 	git clone --depth=1 --no-single-branch $(PLENARY_URL) $(PLENARY_DIR)