Fix double-underscore shader conformance tests

[yudshj]

Summary

• make the double-underscore test use a valid vertex-to-fragment varying
• remove __foo and foo__bar from the reserved-word rejection list

Rationale

PR #3761 changed the WebGL 1.0 language so defining an identifier containing two consecutive underscores does not itself produce an error, and added shader-with-double-underscore.html to exercise that behavior.

Two CTS details currently obscure that behavior:

1. The fragment shader in the new test declares an attribute, which is not valid in an ESSL 1.00 fragment shader. The test therefore fails independently of the identifier under test.
2. shader-with-reserved-words.html still expects __foo and foo__bar to be rejected, contradicting the behavior established by Allow double-underscore e.g. foo__bar in shader identifiers. #3761.

This change gives the test a valid varying interface and removes the two stale reserved-word cases. Implementations that do not support the updated double-underscore behavior will continue to fail the dedicated test for the intended reason.

Fixes the CTS follow-up from #3649 and #3761.

Testing

• python3 sdk/tests/py/lint/lint.py -p
• git diff --check

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