GrailsApplication not found when running test in Grails 5

Question

I'm creating a plugin in Grails 5 and Java 11 to perform processes in a static class, but when running the test described below, the following error message appears:

• Error:

java.lang.IllegalArgumentException: GrailsApplication not found
    at org.springframework.util.Assert.notNull(Assert.java:201)
    at grails.util.Holders.getGrailsApplication(Holders.java:129)
    at grails.util.Holders$getGrailsApplication.call(Unknown Source)
    at org.codehaus.groovy.runtime.callsite.CallSiteArray.defaultCall(CallSiteArray.java:47)
    at org.codehaus.groovy.runtime.callsite.AbstractCallSite.call(AbstractCallSite.java:125)
    at org.codehaus.groovy.runtime.callsite.AbstractCallSite.call(AbstractCallSite.java:130)

• Example scenario:

---
configuration.value: "ABC"

import grails.util.Holders

class SimpleClass {

    static String simpleMethod() {
        return Holders.getGrailsApplication().getConfig().getProperty("configuration.value")
    }

}

import spock.lang.Specification

class SomeTest extends Specification {

    void "example"() {
        expect:
        SimpleClass.simpleMethod() == "ABC"
    }

}