update unit test key: unit -> unit-tests

[emmyoop]

resolves #8698
#8966 resolves the other half of this ticket

Problem

We don't want the top level yaml key for unit tests to be unit

Solution

Make the top level key unit-tests

Checklist

• I have read the contributing guide and understand what's expected of me
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• This PR has no interface changes (e.g. macros, cli, logs, json artifacts, config files, adapter interface, etc) or this PR has already received feedback and approval from Product or DX
• This PR includes type annotations for new and modified functions

[github-actions]

Thank you for your pull request! We could not find a changelog entry for this change. For details on how to document a change, see the contributing guide.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (aa91ea4) 86.63% compared to head (de98e61) 86.66%.

Additional details and impacted files

@@                       Coverage Diff                       @@
##           unit_testing_feature_branch    #8988      +/-   ##
===============================================================
+ Coverage                        86.63%   86.66%   +0.02%     
===============================================================
  Files                              181      181              
  Lines                            26979    26976       -3     
===============================================================
+ Hits                             23374    23378       +4     
+ Misses                            3605     3598       -7     

Flag	Coverage Δ
integration	83.53% <100.00%> (+0.02%)	⬆️
unit	64.65% <66.66%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
core/dbt/config/project.py	97.42% <ø> (ø)
core/dbt/contracts/graph/model_config.py	92.60% <ø> (ø)
core/dbt/contracts/project.py	97.76% <ø> (ø)
core/dbt/parser/partial.py	93.73% <100.00%> (ø)
core/dbt/parser/schemas.py	92.00% <100.00%> (ø)
core/dbt/parser/unit_tests.py	93.51% <100.00%> (ø)

... and 3 files with indirect coverage changes

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