D3D9: Make some test functions static

[H. Verbeet]

Changelog:
  - Make some test functions static
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diff --git a/dlls/d3d9/tests/device.c b/dlls/d3d9/tests/device.c
index 5859d1a..0984aab 100644
--- a/dlls/d3d9/tests/device.c
+++ b/dlls/d3d9/tests/device.c
@@ -64,7 +64,7 @@ static void check_mipmap_levels(
     if (texture) IUnknown_Release( texture );
 }
 
-void test_mipmap_levels(void) 
+static void test_mipmap_levels(void)
 {
 
     HRESULT               hr;
@@ -103,7 +103,7 @@ void test_mipmap_levels(void) 
     DestroyWindow( hwnd );
 }
 
-void test_swapchain(void)
+static void test_swapchain(void)
 {
     HRESULT                      hr;
     HWND                         hwnd               = NULL;
@@ -247,7 +247,7 @@ void test_swapchain(void)
     DestroyWindow( hwnd );
 }
 
-void test_refcount(void)
+static void test_refcount(void)
 {
     HRESULT                      hr;
     HWND                         hwnd               = NULL;