[Select to Speak] Update imports in testing for mv3 compatibility

As noted in crrev.com/c/5274278, the previous solution for importing
code in tests no longer works with manifest v3. This updates the tests
for the Select to Speak extension to the new method.

AX-Relnotes: n/a.
Bug: b/260590502
Change-Id: I2e4d1fe5c7e1dd50bd9349dde3ce4eea5adf0feb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5282853
Commit-Queue: Anastasia Helfinstein <anastasi@google.com>
Reviewed-by: Katie Dektar <katie@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1259554}