Refactor wimpy feature (#2507)

- Make it a common parameter in harness using either '-w', '--wimpy' or
'CL_WIMPY_MODE' environment variable.
- Remove all test specific wimpy variable.

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Co-authored-by: Kévin Petit <kpet@free.fr>

test_conformance/conversions/basic_test_conversions.h

@@ -80,7 +80,6 @@ extern int gHasDouble;
 extern int gTestDouble;
 extern int gHasHalfs;
 extern int gTestHalfs;
-extern int gWimpyMode;
 extern int gWimpyReductionFactor;
 extern int gSkipTesting;
 extern int gMinVectorSize;