Fix browser.test_emscripten_async_wget2.

Need to include <stdarg.h> to use va_start / va_end.
Fix browser.test_float_tex.

This should be getting the size of a float, not a uint.
Include stdlib.h when using malloc().
_GNU_SOURCE definition not needed.
Should be fopen, not open.
Merge pull request #1551 from waywardmonkeys/fix_browser_test_issues

Fix browser test issues