We should respect the value of the PHPUNIT_LONGTEST constant in the
report source stress tests, as it was discovered after 165e26fa that
the default configuration of some CIs (GHA) was not sufficient to run
them.
Hence, use the PHPUNIT_LONGTEST to determine whether to execute said
tests. Note this constant is enabled on internal CI.
Implement stress tester methods for iterating over report columns,
aggregation and conditions. Assert that each works correctly in
isolation, and when used in conjunction with other columns.
Paul Holden