dev/core#880: Fix typo in DROP table query.

[mfb]

Overview

The DROP table query in the utf8mb4 is missing its $conn argument. Hat tip johnk: https://lab.civicrm.org/dev/core/issues/880#note_16365

Before

utf8mb4 test fails if run again because table already exists.

After

Drop table if it exists; properly drop table after running test.

[civibot]

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[seamuslee001]

PR looks right Merging

[kcristiano]

@seamuslee001 Could this be considered for 5.13 as well?