Remove support for libmysql-client from mysqli test suite

Since mysqli can no longer be built against libmysql-client, there is
no longer the need to distinguish.

While we're at

Remove support for libmysql-client from mysqli test suite

Since mysqli can no longer be built against libmysql-client, there is
no longer the need to distinguish.

While we're at it, we also drop the superfluous is_object() checks.

Closes GH-9652.

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Port skipif.inc files to EXTENSIONS

Fixed bug #67983

We need to check the BIT case first, otherwise it will get skipped
in INT_AND_FLOAT_NATIVE mode.