Halve test erofs image size^2

With 4M image size, tests finish in ~55-70 seconds, test timeout is 60s.
So there's still some time outs happening. Reduce image size again.

Test: th
Bug: 249254457
Change-Id: I67e03e65f0831ac49b196598bd316b05dd132fac
diff --git a/sample_images/generate_test_erofs_images.sh b/sample_images/generate_test_erofs_images.sh
index 68d7684..a810529 100755
--- a/sample_images/generate_test_erofs_images.sh
+++ b/sample_images/generate_test_erofs_images.sh
@@ -36,7 +36,7 @@
   truncate -s 1M ${fs_root}/dir1/dir2/file4
   touch ${fs_root}/dir1/dir2/dir123/empty
   cp ${delta_generator} ${fs_root}/delta_generator
-  truncate -s 4M ${fs_root}/delta_generator
+  truncate -s 1M ${fs_root}/delta_generator
   echo "PAYLOAD_MINOR_VERSION=1234" > ${fs_root}/etc/update_engine.conf
   truncate -s 16M ${fs_root}/dir1/dir2/dir123/chunks_of_zero
 fi