build_packages: Install test dependencies for given packages.

Ensure test dependencies are also installed during the build_packages
step. This avoids emerging test dependencies with FEATURES=test during
ebuild testing.

BUG=b:187795989
TEST=./build_packages --board eve --skip_board_setup
chromeos-base/lorgnette

Change-Id: Iffd4c372f04949546d68e29ef93ed7d631ca6991
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosutils/+/2887549
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Sean McAllister <smcallis@google.com>
Tested-by: Navil Perez <navil@google.com>
Commit-Queue: Navil Perez <navil@google.com>
diff --git a/build_packages b/build_packages
index e82b663..540ba59 100755
--- a/build_packages
+++ b/build_packages
@@ -217,7 +217,7 @@
 
 
 # Setup all the emerge command/flags.
-EMERGE_FLAGS=( -uDNv --backtrack=30 --newrepo )
+EMERGE_FLAGS=( -uDNv --backtrack=30 --newrepo --with-test-deps y )
 
 EMERGE_CMD=(
   "${CHROMITE_BIN}/parallel_emerge"