Optimize context-creation-and-destruction test

This test is very long-running, and taking out the unnecessary Uint8Array
construction on every iteration reduced run time by roughly 5% on a
high-end workstation. Additionally, query maximum texture size only once
for some more marginal gains. The changes have no significant effect on
test coverage and are also done to the 1.0.2 version of the test.
Merge pull request #424 from NVIDIA/optimize-context-creation-and-destruction

Optimize context-creation-and-destruction test
Optimize context-creation-and-destruction test

This test is very long-running, and taking out the unnecessary Uint8Array
construction on every iteration reduced run time by roughly 5% on a
high-end workstation. Additionally, query maximum texture size only once
for some more marginal gains. The changes have no significant effect on
test coverage and are also done to the 1.0.2 version of the test.
Moved common test code to iterable-test.js, ditched iFrame approach
Merging Olli's optimization