Fix System.nanoTime definition (#3543)

While redesigning builtins a bug was accidentally introduced.
Stumbled upon accidentally while trying to run `test/Benchmarks`.

engine/runtime/src/main/java/org/enso/interpreter/runtime/system/System.java

@@ -32,8 +32,8 @@ public class System {
 
   @Builtin.Method(description = "Gets the nanosecond resolution system time.")
   @CompilerDirectives.TruffleBoundary
-  public static void nanoTime() {
-    java.lang.System.nanoTime();
+  public static long nanoTime() {
+    return java.lang.System.nanoTime();
   }
 
   @Builtin.Method(description = "Exits the process, returning the provided code.")

test/Tests/src/Main.enso

@@ -59,6 +59,7 @@ import project.Runtime.Lazy_Generator_Spec
 import project.System.File_Spec
 import project.System.Process_Spec
 import project.System.Reporting_Stream_Decoder_Spec
+import project.System.System_Spec
 
 main = Test.Suite.run_main <|
     Any_Spec.spec
@@ -112,3 +113,4 @@ main = Test.Suite.run_main <|
     Vector_Spec.spec
     Statistics_Spec.spec
     Warnings_Spec.spec
+    System_Spec.spec

test/Tests/src/System/System_Spec.enso

@@ -0,0 +1,9 @@
+from Standard.Base import all
+
+import Standard.Base.System
+import Standard.Test
+
+spec = Test.group "System" <|
+    Test.specify "should provide nanosecond timer" <|
+        result = System.nano_time
+        (result > 0).should_equal True