Add tests to verify java versions on CI

src/compatTest/kotlin/io/github/gradlenexus/publishplugin/JavaVersionInCompatTest.kt

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+package io.github.gradlenexus.publishplugin
+
+import org.junit.jupiter.api.Test
+import org.junit.jupiter.api.fail
+
+class JavaVersionInCompatTest {
+
+    @Test fun test() {
+        fail(System.getProperty("java.version").toString())
+    }
+}

src/e2eTest/kotlin/io/github/gradlenexus/publishplugin/e2e/JavaVersionInE2ETest.kt

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+package io.github.gradlenexus.publishplugin.e2e
+
+import org.junit.jupiter.api.Test
+import org.junit.jupiter.api.fail
+
+class JavaVersionInE2ETest {
+
+    @Test fun test() {
+        fail(System.getProperty("java.version").toString())
+    }
+}

src/test/kotlin/io/github/gradlenexus/publishplugin/JavaVersionUnitTest.kt

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+package io.github.gradlenexus.publishplugin
+
+import org.junit.jupiter.api.Test
+import org.junit.jupiter.api.fail
+
+class JavaVersionUnitTest {
+
+    @Test fun test() {
+        fail(System.getProperty("java.version").toString())
+    }
+}