Bump assertj-core from 3.18.1 to 3.19.0 in /core

Bumps [assertj-core](https://github.com/assertj/assertj-core) from 3.18.1 to 3.19.0.
- [Release notes](https://github.com/assertj/assertj-core/releases)
- [Commits](assertj/assertj@assertj-core-3.18.1...assertj-core-3.19.0)

Signed-off-by: dependabot[bot] <support@github.com>

core/build.gradle

@@ -217,7 +217,7 @@ dependencies {
     testCompile files('testlib/repo/fakejar/fakejar/0/fakejar-0.jar')
 
     testCompile 'org.rnorth.visible-assertions:visible-assertions:2.1.2'
-    testCompile 'org.assertj:assertj-core:3.18.1'
+    testCompile 'org.assertj:assertj-core:3.19.0'
     testCompile project(':test-support')
 
     jarFileTestCompileOnly "org.projectlombok:lombok:${lombok.version}"