Fix tests to use proper task execution instead of exec()

- Replace direct .exec() calls with task action execution
- This triggers @TaskAction execute() method which handles:
  - nuget.exe download if needed
  - Mono wrapping on macOS/Linux
  - Proper argument handling and deduplication
- Fixes 4 out of 5 failing tests on macOS with Mono
- Tests now properly exercise the full task execution lifecycle

Author: gamma

src/test/groovy/com/ullink/NuGetDownloadTest.groovy

@@ -210,6 +210,10 @@ class NuGetDownloadTest {
             nuspecFile = nuspec
         }
 
-        project.tasks.nugetPack.exec()
+        // Execute the task properly to trigger @TaskAction execute() method
+        // This ensures nuget.exe download and Mono wrapping are handled correctly
+        project.tasks.nugetPack.actions.each { action ->
+            action.execute(project.tasks.nugetPack)
+        }
     }
 }

src/test/groovy/com/ullink/NuGetPluginTest.groovy

@@ -49,7 +49,10 @@ class NuGetPluginTest {
             nuspecFile = nuspec
         }
 
-        project.tasks.nugetPack.exec()
+        // Execute the task properly to trigger @TaskAction execute() method
+        project.tasks.nugetPack.actions.each { action ->
+            action.execute(project.tasks.nugetPack)
+        }
         assertTrue(project.tasks.nugetPack.packageFile.exists())
     }
 
@@ -78,7 +81,10 @@ class NuGetPluginTest {
             packageVersion = "100.200.300"
         }
 
-        project.tasks.nugetPack.exec()
+        // Execute the task properly to trigger @TaskAction execute() method
+        project.tasks.nugetPack.actions.each { action ->
+            action.execute(project.tasks.nugetPack)
+        }
         def packageFile = project.tasks.nugetPack.packageFile
         assertEquals("bar.100.200.300.nupkg", packageFile.name)
         assertTrue(packageFile.exists())