CircleCI parallelization

* Enable parallelism in .circleci/config.yml
* Add BUSTED_SPEC_FILE to Makefile testfront
* Use it in .ci/test.sh with xargs for test parallelization

NB This is the dumb method of improving test time.
Ideally we want a workflow fan-in/fan-out approach.

.ci/test.sh

@@ -4,4 +4,7 @@ CI_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
 # shellcheck source=/dev/null
 source "${CI_DIR}/common.sh"
 
-make testfront
+pushd koreader-emulator-x86_64-linux-gnu/koreader && {
+    circleci tests glob "spec/front/unit/*_spec.lua" | circleci tests split --split-by=timings --timings-type=filename | xargs -I{} sh -c 'make -C ../.. testfront BUSTED_SPEC_FILE="{}" || exit 255'
+} && popd
+#make testfront

.circleci/config.yml

@@ -7,6 +7,7 @@ jobs:
           EMULATE_READER: 1
           # this is for shellcheck 0.4.5 and lower; can be removed for 0.4.6
           LC_ALL: en_US.UTF8
+    parallelism: 4
     steps:
       - checkout
       - restore_cache:

Makefile

@@ -102,7 +102,7 @@ testfront: $(INSTALL_DIR)/koreader/.busted
 	cd $(INSTALL_DIR)/koreader && ./luajit $(shell which busted) \
 		--sort-files \
 		--no-auto-insulate \
-		-o verbose_print --exclude-tags=notest
+		-o verbose_print --exclude-tags=notest $(BUSTED_SPEC_FILE)
 
 test: $(INSTALL_DIR)/koreader/.busted
 	$(MAKE) -C $(KOR_BASE) test