ewaol-tests: Add the variables replaced via envsubst as vardeps

Variables which are inserted via envsubst do not get detected as build
dependences by default, so modifications do not retrigger the associated build
tasks. Therefore, this patch adds the variables explicitly as vardeps.

Issue-Id: SCM-4394
Signed-off-by: Richard Neill <richard.neill@arm.com>
Change-Id: I27a40a273ae81e31e8be0dbe2c89da5ded9e0dfa