Makefile: Add gentestcases target

This calls gen_test_cases.sh in the proper environment.

Also, prevent gen_test_cases.sh from overriding test_file, to
provide stable signature (and avoid large git diff for no reason).

BRANCH=none
BUG=chromium:684354
TEST=make gentestcases -j8; git diff => no changes

Change-Id: I556285fd1a07a4d84f4ebd3fd7881ae06743716e
Reviewed-on: https://chromium-review.googlesource.com/439064
Commit-Ready: Nicolas Boichat <drinkcat@chromium.org>
Tested-by: Nicolas Boichat <drinkcat@chromium.org>
Reviewed-by: Randall Spangler <rspangler@chromium.org>

Makefile

@@ -1379,6 +1379,11 @@ runtests: test_setup test_targets
 genkeys: utils test_setup
 	tests/gen_test_keys.sh
 
+# Generate test cases
+.PHONY: gentestcases
+gentestcases: utils test_setup
+	tests/gen_test_cases.sh
+
 # Generate test cases for fuzzing
 .PHONY: genfuzztestcases
 genfuzztestcases: utils test_setup

tests/gen_test_cases.sh

@@ -35,6 +35,10 @@ function generate_test_signatures {
 # Generate a file with random bytes for signature tests.
 function generate_test_file {
   echo "Generating test file..."
+  if [ -f "${TEST_FILE}" ]; then
+    echo "(skipping, file already exists)"
+    return
+  fi
   dd if=/dev/urandom of=${TEST_FILE} bs=${TEST_FILE_SIZE} count=1
 }