Fix the metadata for webgl tests.

These are reftests, not tests using the test harness.

CC #8414.  (This fixes the immediate issue, but we should also fix the
error message.)

tests/wpt/mozilla/meta/mozilla/webgl/clearcolor.html.ini

@@ -1,4 +1,4 @@
 [clearcolor.html]
-  type: testharness
+  type: reftest
   disabled:
     if os == "linux": https://github.com/servo/servo/issues/7931

tests/wpt/mozilla/meta/mozilla/webgl/draw_arrays_simple.html.ini

@@ -1,5 +1,5 @@
 [draw_arrays_simple.html]
-  type: testharness
+  type: reftest
   disabled:
     if os == "linux": https://github.com/servo/servo/issues/7931
     if os == "mac": https://github.com/servo/servo/issues/8486

tests/wpt/mozilla/meta/mozilla/webgl/tex_image_2d_canvas.html.ini

@@ -1,5 +1,5 @@
 [tex_image_2d_canvas.html]
-  type: testharness
+  type: reftest
   disabled:
     if os == "linux": https://github.com/servo/servo/issues/7931
     if os == "mac": https://github.com/servo/servo/issues/8486

tests/wpt/mozilla/meta/mozilla/webgl/tex_image_2d_canvas2d.html.ini

@@ -1,5 +1,5 @@
 [tex_image_2d_canvas2d.html]
-  type: testharness
+  type: reftest
   disabled:
     if os == "linux": https://github.com/servo/servo/issues/7931
     if os == "mac": https://github.com/servo/servo/issues/8486

tests/wpt/mozilla/meta/mozilla/webgl/tex_image_2d_canvas_no_context.html.ini

@@ -1,4 +1,4 @@
 [tex_image_2d_canvas_no_context.html]
-  type: testharness
+  type: reftest
   disabled:
     if os == "linux": https://github.com/servo/servo/issues/7931

tests/wpt/mozilla/meta/mozilla/webgl/tex_image_2d_simple.html.ini

@@ -1,5 +1,5 @@
 [tex_image_2d_simple.html]
-  type: testharness
+  type: reftest
   disabled:
     if os == "linux": https://github.com/servo/servo/issues/7931
     if os == "mac": https://github.com/servo/servo/issues/8486