Reduce AbstractFFTs import latency

[Beforerr]

This removes the TestUtils.__init__ error-hint hook, which made using AbstractFFTs spend most of its time compiling even though the hook itself reported as 0.0 ms.

Instead, the test helpers now have small fallback methods that keep the friendly "Did you forget to load Test?" message. Once Test is loaded, the extension methods still take over.

On my machine, the import time dropped from about 75 ms to about 6 ms.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.73%. Comparing base (65b7fcb) to head (7f8be10).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/TestUtils.jl	0.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #156      +/-   ##
==========================================
- Coverage   94.81%   93.73%   -1.09%     
==========================================
  Files           5        5              
  Lines         444      447       +3     
==========================================
- Hits          421      419       -2     
- Misses         23       28       +5     

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[stevengj]

LGTM.