Bump assertj-core from 3.14.0 to 3.15.0 in /core

Bumps [assertj-core](https://github.com/joel-costigliola/assertj-core) from 3.14.0 to 3.15.0.
- [Release notes](https://github.com/joel-costigliola/assertj-core/releases)
- [Commits](assertj/assertj@assertj-core-3.14.0...assertj-core-3.15.0)

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

core/build.gradle

@@ -115,7 +115,7 @@ dependencies {
     // Synthetic JAR used for MountableFileTest and DirectoryTarResourceTest
     testCompile files('testlib/repo/fakejar/fakejar/0/fakejar-0.jar')
 
-    testCompile 'org.assertj:assertj-core:3.14.0'
+    testCompile 'org.assertj:assertj-core:3.15.0'
     testCompile project(':test-support')
 
     jarFileTestCompileOnly "org.projectlombok:lombok:${lombok.version}"