feat(#5): add puzzle for javac method if we are running on JRE

src/main/java/com/yegor256/Jhome.java

@@ -81,6 +81,10 @@ public Path java() {
     /**
      * Find the {@code javac} binary.
      * @return The path of it
+     * @todo #5 The javac binary might not be available in JRE.
+     *  We should check if we are running in JDK or JRE and somehow decide what to
+     *  return in this case or even throw an exception in case of JRE. Maybe an
+     *  {@code Optional<Path>} is also a solution here.
      */
     public Path javac() {
         return this.home.resolve("bin").resolve("javac");

src/test/java/com/yegor256/JhomeTest.java

@@ -28,6 +28,8 @@
 import org.hamcrest.Matchers;
 import org.hamcrest.io.FileMatchers;
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.condition.EnabledForJreRange;
+import org.junit.jupiter.api.condition.JRE;
 
 /**
  * Test case for {@link Jhome}.
@@ -61,6 +63,7 @@ void findsRealFile() {
     }
 
     @Test
+    @EnabledForJreRange(min = JRE.JAVA_9)
     void findsJavac() {
         final Path javac = new Jhome().javac();
         MatcherAssert.assertThat(