Failures caused by textlimit/textsize on freedts mssql

The unit test core_backup_moodle2_testcase::test_restore_legacy_availability() was causing a segfault for me on mssql.

The fix I put in place was adding to php.ini:
mssql.textlimit = 20971520
mssql.textsize = 20971520

(As specified on https://docs.moodle.org/27/en/Installing_MSSQL_for_PHP#Installation_overview )

I don't know how common an issue this is in production, but it strikes me that:
a) We could warn about these values not being correct in unit tests
b) If its a big problem for production sites, we could also warn about it on the admin page.