Removes unused module junit-toolbox

[froque]

This module is not used since commit ed01433

Module junit-toolbox depends on mockito-core 1.9.5 which makes unit test
JdbcDatabaseContainerTest fail on Java 16 or greater.

Exception on master with Java 16:

$ JAVA_HOME=/home/froque/.jdks/adopt-openjdk-16.0.2 ./gradlew :jdbc:test --tests org.testcontainers.containers.JdbcDatabaseContainerTest 
Configuration on demand is an incubating feature.

> Task :jdbc:test FAILED

Gradle Test Executor 1 > org.testcontainers.containers.JdbcDatabaseContainerTest > anExceptionIsThrownIfJdbcIsNotAvailable STARTED

Gradle Test Executor 1 > org.testcontainers.containers.JdbcDatabaseContainerTest > anExceptionIsThrownIfJdbcIsNotAvailable FAILED
    java.lang.AssertionError: 
    Expecting actual throwable to be an instance of:
      java.lang.IllegalStateException
    but was:
      java.lang.ExceptionInInitializerError
        at org.mockito.cglib.core.KeyFactory$Generator.generateClass(KeyFactory.java:167)
        at org.mockito.cglib.core.DefaultGeneratorStrategy.generate(DefaultGeneratorStrategy.java:25)
        at org.mockito.cglib.core.AbstractClassGenerator.create(AbstractClassGenerator.java:217)
        ...(61 remaining lines not displayed - this can be changed with Assertions.setMaxStackTraceElementsDisplayed)
        at org.testcontainers.containers.JdbcDatabaseContainerTest.anExceptionIsThrownIfJdbcIsNotAvailable(JdbcDatabaseContainerTest.java:20)

1 test completed, 1 failed

FAILURE: Build failed with an exception.

[kiview]

Merged, thanks for your contribution @froque 🙂