[infra] Restore 'test' subdirectories to the Debian build tarball

Boringssl build has some dependencies on files in its test directory.

TBR=athom@google.com

Change-Id: I4c7b739481459658940919a782238497cb2ccaf1
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/196326
Reviewed-by: William Hesse <whesse@google.com>

tools/linux_dist_support/create_tarball.py

@@ -60,7 +60,6 @@ ignoredDirs = [
     'docs',
     'fuchsia',
     'parser_testcases',
-    'test',
     'testcases',
     'tests',
 ]