Bump Markdown timeout from 3 to 6 seconds in test

We've been seeing increased flaky failures due to the timeout exceeding 3 seconds. With the additional regexp in e38e2adc, we see that performance has gotten worse:

1.376477   0.005843   1.382320 (  1.423449)

Previously it was:

0.566795 0.000933 0.567728 ( 0.569076)

Closes #35269 (closed)