Do not leak state between adjacent XFB tests

Unbind the buffer from the COPY_WRITE_BUFFER target
so that it is bound only as XFB for the next test.

sdk/tests/conformance2/transform_feedback/simultaneous_binding.html

@@ -303,6 +303,7 @@
   wtu.glErrorShouldBe(gl, gl.INVALID_OPERATION, "copyBufferSubData with double bound buffer");
   gl.copyBufferSubData(gl.COPY_READ_BUFFER, gl.COPY_WRITE_BUFFER, 0, 0, 1);
   wtu.glErrorShouldBe(gl, gl.INVALID_OPERATION, "copyBufferSubData with double bound buffer");
+  gl.bindBuffer(gl.COPY_WRITE_BUFFER, null);
 
   debug("<hr/>Test that rejected operations do not change the bound buffer size");