Add outputs to generateKotlinCompilerTests

[JavierSegoviaCordoba]

Build scan:

Path	:kotlin-compiler-extensions:generateKotlinCompilerTests	
Type	org.gradle.api.tasks.JavaExec	
This task is on the critical path.	
Started after	1.918s	
Duration	0.469s	
The task was not up-to-date because it did not have any declared outputs.	
Not cacheable	Gradle would require more information to cache this task.	
Java toolchain	Azul Zulu 17.0.7+7-LTS (amd64)	
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The outputs should be the test-gen directory, so projectDir.resolve("test-gen"), or something similar with ProjectLayout APIs