Introduce fuzz target to minimize failing programs

[techraed]

Problem to Solve

Corpora can include only one failing program, but ten are generated. If we run cargo fuzz tmin to minimize test input, it will minimize not program, but the corpora itself, which is inefficient. The failing program must be "minimized".

Possible Solution

Introduce a separate fuzz target which only runs a specific extrinsic with a specific program and execute cargo fuzz tmin on that target.