Convert ApproxFunBaseTest to a package extension

[jishnub]

After this, downstream packages don't need to depend explicitly on ApproxFunBaseTest, but may instead depend on the internal module ApproxFunBase.TestUtils, which is equivalent in functionality. The advantage is that the methods are defined in a package extension, so the package does not need to be maintained separately.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 166 lines in your changes missing coverage. Please review.

Project coverage is 27.86%. Comparing base (023b1a4) to head (31d9116). Report is 12 commits behind head on master.

Files	Patch %	Lines
ext/ApproxFunBaseTestExt.jl	0.00%	166 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #657 +/- ## =========================================== - Coverage 75.20% 27.86% -47.35% =========================================== Files 79 80 +1 Lines 8445 8575 +130 =========================================== - Hits 6351 2389 -3962 - Misses 2094 6186 +4092 ```

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