Flaky Tests: split up dbt show test class

[emmyoop]

resolves #8470
docs dbt-labs/docs.getdbt.com/#

Problem

TestShow::test_numeric_values & TestShow::test_numeric_values_with_nulls are flaky

Solution

Split up tests into separate classes

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

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: -0.05% ⚠️

Comparison is base (b39eeb3) 86.61% compared to head (fc6ed96) 86.56%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8610      +/-   ##
==========================================
- Coverage   86.61%   86.56%   -0.05%     
==========================================
  Files         174      174              
  Lines       25590    25590              
==========================================
- Hits        22164    22152      -12     
- Misses       3426     3438      +12     

Flag	Coverage Δ
integration	83.36% <ø> (-0.13%)	⬇️
unit	65.11% <ø> (ø)

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see 2 files with indirect coverage changes

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