[BUG] test isolation in XQSuite

[line-o]

Describe the bug

While working on #6060 I noticed that tests, specifically the tests calling fn:transform are inadequately isolated against each other.
I want to stress that this is either an issue within fn:transform or the xqsuite test runner or both. See also this issue with main modules that evaluate to a deferred function call which leads to undeterministic test case failures.

I created a gist with the necessary information in order to keep this issue description brief.

The gist (of the gist)

declare %test:assertEquals("<a />") function xqsuite-test-isolation:test-1() {
    transform($options)?output
};

declare %test:assertEquals("<a />") function xqsuite-test-isolation:test-2() {
    transform($options)?output
};

test-2 will fail because it receives 2 nodes as a result instead of just one. The first node being the result of test-1

Expected behavior

The tests to be isolated enough so that results from a previous test can never influence a later test case.

To Reproduce

Store https://gist.github.com/line-o/4731ed367f437be9cd036b152a18d340#file-xqsuite-test-isolation-xqm in an instance of exist 7.0.0-SNAPSHOT or 6.4.1-SNAPSHOT and run them.

While the first test that is executed will pass all following ones do not. They all fail with a cardinality error.

Add the delivery-format option or the additional empty-sequence to see that this mitigates the issue entirely.

Context (please always complete the following information)

• Build: eXist-7.0.0-SNAPSHOT (1d5848b)
• Java: 21.0.8 (Eclipse Adoptium)
• OS: Linux 6.12.54-linuxkit (aarch64)

Additional context

• How is eXist-db installed? docker
• Any custom changes in e.g. conf.xml? none