[NFC/Test framework] Fix failing test when run as part of whole suite

The utf8 install test was passing locally but failing in CI. But it was passing locally because I was only running that one test. When run as part of the whole suite, what happens is that because the civi testing framework does NOT clear the list of installed extensions when it repopulates the database (for ?reasons?), it sees it as already being installed when it gets to this test so doesn't run the installer code, but that's part of what this test is testing!