LU-17734 build: Debian: oblige --disable-tests if asked

LU-17734 build: Debian: oblige --disable-tests if asked

Do not disable tests by default for debian-based builds, but permit
users to disable them if they choose by passing in --disable-tests.

Test-Parameters: trivial
Signed-off-by: Ellis Wilson <elliswilson@microsoft.com>
Change-Id: I90088e6e95fa9e46ae063dfc061a324293fde9a2
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/54764
Reviewed-by: Timothy Day <timday@amazon.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>