Add mssql migration tests (needs #6823)

[zeripath]

This PR adds Migration tests for MSSQL.

It will fail at present due to the bug in v78.go and therefore requires #6823

It will also fail at present because of a bug in v85.go and requires #6851

[zeripath]

This could be backported to 1.8 easily once #6823 is merged and backported as #6727 has already been backported.

[zeripath]

Also requires #6823

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@722a2bd). Click here to learn what that means.
The diff coverage is 0%.

@@            Coverage Diff            @@
##             master    #6852   +/-   ##
=========================================
  Coverage          ?   41.14%           
=========================================
  Files             ?      425           
  Lines             ?    58484           
  Branches          ?        0           
=========================================
  Hits              ?    24061           
  Misses            ?    31238           
  Partials          ?     3185

Impacted Files	Coverage Δ
models/migrations/migrations.go	1.52% <0%> (ø)

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