fix(test): use a random available port on http test

Instead of using the default mojo port (2000) during http test, find the
first available port on the host an use it during tests.

src/test/groovy/org/asciidoctor/maven/test/AsciidoctorHttpMojoTest.groovy

@@ -26,6 +26,8 @@ class AsciidoctorHttpMojoTest extends Specification {
             System.setOut(new PrintStream(newOut))
             System.setIn(newIn)
 
+            def httpPort = new AsciidoctorMojoTestHelper().availablePort
+
             def content = new File(srcDir, "content.asciidoc")
             content.withWriter{ it <<
                 '''Document Title
@@ -35,6 +37,7 @@ class AsciidoctorHttpMojoTest extends Specification {
 
             def mojo = new AsciidoctorHttpMojo()
             mojo.backend = 'html5'
+            mojo.port = httpPort
             mojo.sourceDirectory = srcDir
             mojo.outputDirectory = outputDir
             mojo.headerFooter = true
@@ -52,7 +55,7 @@ class AsciidoctorHttpMojoTest extends Specification {
             }
 
         when:
-            def html = 'http://localhost:2000/content'.toURL().text
+            def html = "http://localhost:${httpPort}/content".toURL().text
 
         then:
             assert html.contains('This is test, only a test')
@@ -79,6 +82,8 @@ class AsciidoctorHttpMojoTest extends Specification {
             def newOut = new DoubleOuputStream(originalOut)
             def newIn = new PrefilledInputStream('exit\r\n'.bytes, inputLatch)
 
+            def httpPort = new AsciidoctorMojoTestHelper().availablePort
+
             System.setOut(new PrintStream(newOut))
             System.setIn(newIn)
 
@@ -91,6 +96,7 @@ class AsciidoctorHttpMojoTest extends Specification {
 
             def mojo = new AsciidoctorHttpMojo()
             mojo.backend = 'html5'
+            mojo.port = httpPort
             mojo.sourceDirectory = srcDir
             mojo.outputDirectory = outputDir
             mojo.headerFooter = true
@@ -108,7 +114,7 @@ class AsciidoctorHttpMojoTest extends Specification {
             }
 
         when:
-            def html = 'http://localhost:2000/'.toURL().text
+            def html = "http://localhost:${httpPort}/".toURL().text
 
         then:
             assert html.contains('DEFAULT')

src/test/groovy/org/asciidoctor/maven/test/AsciidoctorMojoTestHelper.groovy

@@ -0,0 +1,10 @@
+package org.asciidoctor.maven.test
+
+class AsciidoctorMojoTestHelper {
+    def getAvailablePort() {
+        ServerSocket socket = new ServerSocket(0)
+        def port = socket.getLocalPort()
+        socket.close()
+        return port
+    }
+}