Add integration test for mill-jvm-version: system (#5740)

Follow up of #5705

Author: lihaoyi

integration/feature/system-java/resources/build.mill

@@ -0,0 +1,10 @@
+package build
+import mill._
+
+def printJavaVersion() = Task.Command {
+  println(scala.util.Properties.javaVersion)
+}
+
+def printJavaHome() = Task.Command {
+  println(scala.util.Properties.javaHome)
+}

integration/feature/system-java/src/SystemJavaTests.scala

@@ -0,0 +1,26 @@
+package mill.integration
+
+import mill.testkit.UtestIntegrationTestSuite
+import utest.*
+
+object SystemJavaTests extends UtestIntegrationTestSuite {
+  override def propagateJavaHome = false
+  val tests: Tests = Tests {
+
+    test("header") - integrationTest { tester =>
+      val defaultVersion = tester.eval("printJavaVersion")
+      assert(defaultVersion.out.contains("21.0.8"))
+
+      val defaultHome = tester.eval("printJavaHome")
+      tester.modifyFile(
+        tester.workspacePath / "build.mill",
+        s => "//| mill-jvm-version: system\n" + s
+      )
+      val updatedHome = tester.eval("printJavaHome")
+      pprint.log(updatedHome.out)
+      pprint.log(defaultHome.out)
+      assert(updatedHome.out != defaultHome.out)
+    }
+
+  }
+}