RGB10_A2 invalid for DOM uploads per spec, but required in conformance tests

"When the data source is a DOM element [...] each channel's representation is an unsigned integer type of at least 8 bits. Converting such representation to signed integers or unsigned integers with more bits is not clearly defined. [...] Therefore, only converting to unsigned integer of at most 8 bits, half float, or float is allowed."

I'm triaging our test failures, and Firefox is failing these tests:
"Error: WebGL warning: texImage2D: Format or type is invalid for DOM sources."

Any objections to removing these tests? (and replacing with negative tests?)

@kenrussell