JUnit: Upgrade to JUnit 5.4.0

Currently, JUnit 5.1.0 is used to write JUnit 5 styled tests.

JUnit 5.1.0 does not support creating Temporary Folders natively, and
relies on having to create a custom TemporaryFolder extension or
relying on JUnit-Pioneer library [1]. Both of these solutions make it
complicated to implement and understand. JUnit 5.4.0 is the first
version of JUnit that supports native TemporaryFolder extension [2].

Hence, let's use JUnit 5.4.0 instead to reduce the complexity of
implementing TemporaryFolder.

[1]
https://medium.com/@GalletVictor/migration-from-junit-4-to-junit-5-d8fe38644abe
[2] junit-team/junit5#1247

build.gradle

@@ -43,7 +43,7 @@ test {
 
 dependencies {
     String testFxVersion = '4.0.12-alpha'
-    String jUnitVersion = '5.1.0'
+    String jUnitVersion = '5.4.0'
 
     implementation group: 'com.fasterxml.jackson.core', name: 'jackson-databind', version: '2.7.0'
     implementation group: 'com.fasterxml.jackson.datatype', name: 'jackson-datatype-jsr310', version: '2.7.4'