TE-468 JS test improvement: test for attributes specifically

This test is failing in a different test environment (Solano) because
the built html has reversed the order of the width and height attributes
from what we are expecting. This change focuses the assertion to the
pieces that are important (as noted in the unchanged comment above
the assertion).
Merge pull request #5169 from edx/benp/solano2-jstest-failure

TE-468 JS test improvement: test for attributes specifically