Allow modules to opt-in to strict variables

Strict variables can be disabled in spec tests by setting
`STRICT_VARIABLES` to `no`. This is done by default for existing
modules, since many of them have strict variable errors in third-party
dependencies.

This patch allows us to opt-in to strict variables in modules which
support it.
1 job for strict_variables_optin in 39 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #38805
build

00:00:39