Bump assertj-core from 3.12.2 to 3.13.2 in /core (#1789)

Author: dependabot-preview[bot]

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:3.12.2'
+    testCompile 'org.assertj:assertj-core:3.13.2'
     testCompile project(':test-support')
 
     jarFileTestCompileOnly "org.projectlombok:lombok:${lombok.version}"