How to access resources during local unit test?

Is there a way to access resources during unit test ? I tried many possibilities found in StackOverflow, but I failed with all of them.

The last I tried (match to the code below) give me this error for the task testReleaseUnitTest :

java.lang.IllegalStateException: No instrumentation registered! Must run under a registering instrumentation.
    at androidx.test.platform.app.InstrumentationRegistry.getInstrumentation(InstrumentationRegistry.java:45)
    at androidx.test.core.app.ApplicationProvider.getApplicationContext(ApplicationProvider.java:41)
    at fr.alpha.calculator.TestOperationManager.testAddCharacterEmptyOperation(TestOperationManager.java:20)

An instrumentation is created with instrumentation tests, right ? Is it possible to avoid that to make the tests quicker ?

Thanks for reading !

My test class :

package fr.alpha.calculator;

import static org.junit.jupiter.api.Assertions.*;

import org.junit.jupiter.api.function.Executable;
import org.junit.jupiter.api.Test;

import android.content.Context;
import android.content.res.Resources;
import androidx.test.core.app.ApplicationProvider;

import fr.alpha.calculator.OperationManager;

public class TestOperationManager{

    @Test
    public void testAddCharacterEmptyOperation(){

        final Context context = ApplicationProvider.getApplicationContext();
        final Resources res = context.getResources();

        final OperationManager testOperationManager = new OperationManager(res);
            
        assertTrue(testOperationManager.addCharacter('0'), 
            "addCharacter must return true !");

        final OperationManager expectedOperationManager =
            new OperationManager(res);
        expectedOperationManager.setOperation("0");

        assertEquals(expectedOperationManager, testOperationManager, 
            "operation must be equal to \"0\" !");

    }
}

My app/build.gradle file :

apply plugin: 'com.android.application'

android {
    compileSdkVersion 28
    defaultConfig {
        applicationId "fr.alpha.calculator"
        minSdkVersion 22
        targetSdkVersion 28
        versionCode 1
        versionName "1.0"
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }

    compileOptions {
        sourceCompatibility JavaVersion.VERSION_1_8
        targetCompatibility JavaVersion.VERSION_1_8
    }

    testOptions{
        unitTests.all{
            useJUnitPlatform()
        }
    }
}

dependencies {
    implementation 'androidx.constraintlayout:constraintlayout:1.1.3'
    implementation 'androidx.gridlayout:gridlayout:1.0.0'
    implementation 'androidx.appcompat:appcompat:1.1.0'
    implementation 'org.apache.commons:commons-lang3:3.10'
    testImplementation 'org.junit.jupiter:junit-jupiter-api:5.6.2'
    testImplementation 'androidx.test:core:1.2.0'
    testRuntimeOnly 'org.junit.jupiter:junit-jupiter-engine:5.6.2'
}