Trying out Fleet IDE now a bit, but again, missing an important thing: running Kotest tests... how can I run a particular test using gradle test task? What are my options? Is there any docs for this?

You could use test filtering in your Gradle build script like so:

tasks.withType<Test>().configureEach {
    useJUnitPlatform()
    filter {
        listOf<String>(
             "serialization.CommonSerializationTests*",
             "serialization.JvmSerializationTests*",
        ).forEach {
            includeTestsMatching("${rootProject.name}.$it")
        }
    }
}

For BDD, I need to run the test class I'm currently working on... it wouldn't be too practical to mangle with build.gradle.kts for this...

I'm not using Fleet currently. Which plugin are you referring to?

The intellij plugin for kotest.

You'll need the Kotest plugin to run a single test from the IDE without messing with build scripts or test sources (e.g. via the focus indicator "!"). Compatible IntelliJ versions are listed on the plugin marketplace, your EAP may be too new. AFAIK, Fleet doesn't support plugins so far. Maybe you could use the Gradle command line.

That's what I was looking for, the command line, thanks!