Prevent tests from failing with FreeTDS

[bkline]

Some tests have been tweaked, others skipped.

Also fixes a typo in string used to detect the FreeTDS drier.

Closes #1471

[mkleehammer]

I verified this under FreeTDS locally -- version 1.3.17 (what Ubuntu 24.04 ships), against SQL Server 2022. On master the detection typo means none of the FreeTDS skips fire, and the suite segfaults at test_cursor_messages_with_fast_executemany. With this PR the detection is fixed (tdsodbc now matches the libtdsodbc.so driver name), the version gates all fire, and the crash is gone -- the suite goes from a hard crash to 91 passed, 7 skipped.

Two notes:

1. The rename needs a rebase. IS_FREEDTS became IS_FREETDS, but since this PR was opened master picked up another IS_FREEDTS use (in test_tvp_with_nulls, from Avoid heap corruption with NULLs in TVPs (#1450) #1452), and Fix .statistics() issue with Unicode table names #1460 is about to add one more (test_statistics_unicode). Those aren't in this diff, so as-is the merge leaves a dangling IS_FREEDTS and a NameError on import. I'll rebase and finish the rename across all of them.

2. On 1.3.17 one failure remains -- test_context_manager_failure -- but it passes in isolation, so it's a cascade from the FreeTDS "COMMIT with no BEGIN" bug (dataSources() DSN truncation #718) landing on a different test than the conn2 workaround anticipated. The version gates here target 1.5.x, so 1.3.17 is likely just below the floor you validated against -- flagging it in case you want to widen the workaround or document a minimum FreeTDS version.

Thanks @bkline -- big improvement.