[Gradle] Add integration tests for KMP empty and non-test source sets

[Yermukhamed Shakhman (ermawqa-jetbrains)]

Summary

Added MppEmptyTestSourceSetsIT to verify KMP test execution and discovery across empty, non-test, and inherited test source set layouts.

Changes

• Added MppEmptyTestSourceSetsIT covering:
  • No test sources: Verifies :compileTestKotlinJvm and :jvmTest are NO_SOURCE and produce no test results.
  • Sources without tests: Verifies :jvmTest fails on Gradle 9.0+ when compiled classes contain zero @Test methods.
  • Common test discovery & inheritance: Verifies test execution from commonTest and subclass inheritance in jvmTest.
• Added assertNoTestResultsProduced assertion helper in testAssertions.kt.

P.S: keeping it JVM-only is intentional to keep execution fast and lightweight, as KMP source set hierarchy propagation and NO_SOURCE optimizations are target-agnostic. Native targets use different test runner tasks (KotlinNativeHostTest) and require heavy toolchains.

^KQA-3433

[kodee-bot]

Hi! It looks like there are no references to any YT issues in the commit messages.
If the change is non-functional, feel free to ignore the comment :)
Otherwise, please add them as requested in the guideline.

[kotlin-safemerge]

Code Owners

Rule	Owners	Approval
/​libraries/​tools/​kotlin-​gradle-​plugin-​integration-​tests/​	kotlin-build-tools AYastrebov Egorand Tapchicoma antohaby 🌴 Herudaio atyrin (QA) timofeys1 🌴 nav-nav CherepanovAleksei rbbozkurt 🌴 shinkai-tester (QA) 🌴 viakunin levchenkoin (QA) adam-enko ermawqa-jetbrains	✅ Egorand 🔒

PR commands for maintainers

Command	Description	Parameters
/safe-merge	Rebase-merges with automatic fixup commit squashing	--fixup Autosquash fixup commits (on by default)
/safe-squash-merge	Squash-merges with optional commit title/body override	--title Title of the squashed commit --message Body of the squashed commit
/dry-run	Runs the test pipeline with changes rebased on latest master	--retry Retry the CI run on failure
/test-public	Triggers the public test suite without rebasing on latest master	—
/test-private	Triggers the private test suite without rebasing on latest master	—
/codeowners	Triggers code owners check and comment update	—
/fixup	Squashes fixup commits and force pushes the branch	—
/cancel-coordinator	Cancels the merge coordinator currently running for this branch	—

[Egor Andreevich (Egorand)]

Looks good!

/final