Sokol WebGL2 tests (#11781)

As suggested by floooh in

#11771 (comment)

This just adds the 3 WebGL2 ones, which seems like an area we have too
little coverage of.

tests/test_browser.py

@@ -2686,6 +2686,23 @@ def test_webgl2_packed_types(self):
   def test_webgl2_pbo(self):
     self.btest(path_from_root('tests', 'webgl2_pbo.cpp'), args=['-s', 'MAX_WEBGL_VERSION=2', '-lGL'], expected='0')
 
+  @no_firefox('fails on CI likely due to GPU drivers there')
+  @requires_graphics_hardware
+  def test_webgl2_sokol_mipmap(self):
+    self.btest(path_from_root('tests', 'third_party', 'sokol', 'mipmap-emsc.c'), args=['-s', 'MAX_WEBGL_VERSION=2', '-lGL', '-O1'],
+               reference=os.path.join('third_party', 'sokol', 'mipmap-emsc.png'), reference_slack=2)
+
+  @no_firefox('fails on CI likely due to GPU drivers there')
+  @requires_graphics_hardware
+  def test_webgl2_sokol_mrt(self):
+    self.btest(path_from_root('tests', 'third_party', 'sokol', 'mrt-emcc.c'), args=['-s', 'MAX_WEBGL_VERSION=2', '-lGL'],
+               reference=os.path.join('third_party', 'sokol', 'mrt-emcc.png'))
+
+  @requires_graphics_hardware
+  def test_webgl2_sokol_arraytex(self):
+    self.btest(path_from_root('tests', 'third_party', 'sokol', 'arraytex-emsc.c'), args=['-s', 'MAX_WEBGL_VERSION=2', '-lGL'],
+               reference=os.path.join('third_party', 'sokol', 'arraytex-emsc.png'))
+
   def test_sdl_touch(self):
     for opts in [[], ['-O2', '-g1', '--closure', '1']]:
       print(opts)