Gradle: Substitute a library when running tests

Usecase

I'm building a library and it depends on rxjava.

Since rxjava is large, I decided to minify it using proguard and make another module called :custom_rx.

Problem

Since .test() method of RxJava does not need to be at the release, I removed tests for the minified module. However, tests will not pass.

Question

Since test task and release tasks are different, I can tell gradle to use non-modified RxJava for test tasks.

How can I replace a library to another, only when gradle is running tests?

Code I've tried

I used this code to substitute the library on test. But tests still fail:

subprojects {
    project.configurations.all { config ->
        if (config.name.contains("test") || config.name.contains("Test")) {
            resolutionStrategy.dependencySubstitution {
                substitute project(":custom_rx") because "we use a non-minified rxjava for tests" with module("io.reactivex.rxjava2:rxjava:$versions.rxJava")
            }
        }
    }
}