Description:
Some CI jobs differ only in the configured default site (e.g. wikipedia:en
and wikisource:zh). Tests derived from DefaultSiteTestCase should run in
each job because they verify the configured default site.

However, tests with fixed family/code (or a predefined sites
configuration) always run against the same wiki and therefore execute
identical test cases multiple times.

Introduce an environment variable to skip these fixed-site tests in redundant
default-site CI runs. This should reduce CI runtime without reducing overall
test coverage.