Backport: Convert ApproxFunBaseTest to a package extension

[jishnub]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 154 lines in your changes missing coverage. Please review.

Project coverage is 27.37%. Comparing base (391bb31) to head (9f7eba1).
Report is 1 commits behind head on backport_0.8.

❗ Current head 9f7eba1 differs from pull request most recent head 6ba08d7

Please upload reports for the commit 6ba08d7 to get more accurate results.

Files	Patch %	Lines
ext/ApproxFunBaseTestExt.jl	0.00%	154 Missing ⚠️

Additional details and impacted files

@@                Coverage Diff                @@
##           backport_0.8     #658       +/-   ##
=================================================
- Coverage         71.48%   27.37%   -44.11%     
=================================================
  Files                81       82        +1     
  Lines              8623     8255      -368     
=================================================
- Hits               6164     2260     -3904     
- Misses             2459     5995     +3536     

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