Travis: Fix accidental building of 3rd-party dependencies.

Running tests from the top-level Makefile will use the third-party
dependencies from .deps instead of the ones from the Travis cache.

.ci/common/test.sh

@@ -49,11 +49,11 @@ asan_check() {
 }
 
 run_unittests() {
-  ${MAKE_CMD} unittest
+  ${MAKE_CMD} -C "${BUILD_DIR}" unittest
 }
 
 run_functionaltests() {
-  if ! ${MAKE_CMD} ${FUNCTIONALTEST}; then
+  if ! ${MAKE_CMD} -C "${BUILD_DIR}" ${FUNCTIONALTEST}; then
     asan_check "${LOG_DIR}"
     valgrind_check "${LOG_DIR}"
     exit 1
@@ -63,7 +63,8 @@ run_functionaltests() {
 }
 
 run_oldtests() {
-  if ! make oldtest; then
+  ${MAKE_CMD} -C "${BUILD_DIR}" helptags
+  if ! make -C "${TRAVIS_BUILD_DIR}/src/nvim/testdir"; then
     reset
     asan_check "${LOG_DIR}"
     valgrind_check "${LOG_DIR}"

Makefile

@@ -89,7 +89,7 @@ oldtest: | nvim helptags
 	+$(SINGLE_MAKE) -C src/nvim/testdir $(MAKEOVERRIDES)
 
 helptags: | nvim
-	+$(BUILD_CMD) -C build runtime/doc/tags
+	+$(BUILD_CMD) -C build helptags
 
 functionaltest: | nvim
 	+$(BUILD_CMD) -C build functionaltest