Add integration tests subproject: integration-test

* JUnit 5 (API & Engine)
* Tests compile/run as JDK 11+
* Tests require Gradle 4.6+

integration-test/build.gradle

@@ -0,0 +1,22 @@
+plugins {
+    id 'java'
+    id 'java-library'
+    id 'eclipse'
+}
+
+dependencies {
+    implementation project(':bitcoinj-core')
+
+    testImplementation 'org.slf4j:slf4j-jdk14:1.7.36'
+    testImplementation "org.junit.jupiter:junit-jupiter-api:5.8.2"
+    testRuntimeOnly "org.junit.jupiter:junit-jupiter-engine:5.8.2"
+}
+
+sourceCompatibility = 11
+compileJava.options.encoding = 'UTF-8'
+compileTestJava.options.encoding = 'UTF-8'
+javadoc.options.encoding = 'UTF-8'
+
+test {
+    useJUnitPlatform()
+}

settings.gradle

@@ -3,6 +3,8 @@ import org.gradle.api.GradleScriptException
 
 // Minimum Gradle version for build
 def minGradleVersion = GradleVersion.version("4.4")
+// Minimum Gradle version for JUnit5
+def minJunit5GradleVersion = GradleVersion.version("4.6")
 // Minimum Gradle version for builds of JavaFX 11 module
 def minFxGradleVersion = GradleVersion.version("4.10")
 
@@ -34,3 +36,10 @@ if (GradleVersion.current().compareTo(minFxGradleVersion) > 0) {
     project(':wallettemplate').name = 'bitcoinj-wallettemplate'
 }
 
+if (GradleVersion.current().compareTo(minJunit5GradleVersion) >= 0) {
+    System.err.println "Including integration-test because ${GradleVersion.current()}"
+    include 'integration-test'
+    project(':integration-test').name = 'bitcoinj-integration-test'
+} else {
+    System.err.println "Skipping integration-test, requires Gradle ${minJunit5GradleVersion}+, currently running: ${GradleVersion.current()}"
+}