Fix Unit Tests

[papeh]

• Update Deprecated API calls
• Pin to Ubuntu 20.04 until we can get hg back on the path

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This change is 

[github-actions[bot]]

Test Results

       6 files  +       2     321 suites  +202   59m 0s :stopwatch: + 28m 59s    957 tests +       3     898 :heavy_check_mark: +     38    59 :zzz:  - 20  0 :x:  - 15  3 090 runs  +1 990  2 942 :heavy_check_mark: +1 937  148 :zzz: +69  0 :x:  - 16 

Results for commit b1ddc9b2. ± Comparison against base commit aa17de8b.

This pull request removes 2 and adds 5 tests. Note that renamed tests count towards both.

``` CreateWithInstalledHandlers_DefaulthandlerIsNotInMainCollection CreateWithTestHandlerOnly_DefaulthandlerIsNotInTestCollection ``` ``` CreateWithInstalledHandlers_DefaultHandlerIsNotInMainCollection CreateWithTestHandlerOnly_DefaultHandlerIsNotInTestCollection ExternalFileModification_NotifiesIndices_ButSaveDoesNot HandlerShouldProcessOnlyTenMegabyteSizedFiles SaveAndLoad_10KRecords_CompletesQuickly ```

:recycle: This comment has been updated with latest results.