Allow testing only a subset of directories

This can be done by calling TESTDIRS="./report ./probe" make tests

Makefile

@@ -67,7 +67,7 @@ $(SCOPE_EXE) $(RUNSVINIT) lint tests shell prog/static.go: $(SCOPE_BACKEND_BUILD
 		-v $(shell pwd)/.pkg:/go/pkg \
 		--net=host \
 		-e GOARCH -e GOOS -e CIRCLECI -e CIRCLE_BUILD_NUM -e CIRCLE_NODE_TOTAL \
-		-e CIRCLE_NODE_INDEX -e COVERDIR -e SLOW \
+		-e CIRCLE_NODE_INDEX -e COVERDIR -e SLOW -e TESTDIRS \
 		$(SCOPE_BACKEND_BUILD_IMAGE) SCOPE_VERSION=$(SCOPE_VERSION) GO_BUILD_INSTALL_DEPS=$(GO_BUILD_INSTALL_DEPS) $@
 
 else

tools/test

@@ -47,8 +47,15 @@ fi
 
 fail=0
 
-# NB: Relies on paths being prefixed with './'.
-TESTDIRS=( $(git ls-files -- '*_test.go' | grep -vE '^(vendor|prog|experimental)/' | xargs -n1 dirname  | sort -u | sed -e 's|^|./|') )
+if [ -z "$TESTDIRS" ]; then
+    # NB: Relies on paths being prefixed with './'.
+    TESTDIRS=( $(git ls-files -- '*_test.go' | grep -vE '^(vendor|prog|experimental)/' | xargs -n1 dirname  | sort -u | sed -e 's|^|./|') )
+else
+    # TESTDIRS on the right side is not really an array variable, it
+    # is just a string with spaces, but it is written like that to
+    # shut up the shellcheck tool.
+    TESTDIRS=( $(for d in ${TESTDIRS[*]}; do echo "$d"; done) )
+fi
 
 # If running on circle, use the scheduler to work out what tests to run on what shard
 if [ -n "$CIRCLECI" ] && [ -z "$NO_SCHEDULER" ] && [ -x "$DIR/sched" ]; then