Mix of expansion strategies in GrammarCoverageFuzzer

[michaelmera]

Describe the bug
Because the GrammarCoverageFuzzer inherits from the GrammarFuzzer, it still uses the expansion strategy based on costs. This can result in the fuzzer avoiding completely parts of the grammar because it first selects expansions with maximum cost. For a fuzzer that pretends to cover the grammar that is pretty confusing.

To Reproduce
For example, the following code will never produce an 'a':

from fuzzingbook.GrammarCoverageFuzzer import GrammarCoverageFuzzer

grammar = {
    '<start>': ['<A>', '<B>'],
    '<A>': ['a'],
    '<B>': ['b<C>', '<D>'],
    '<C>': ['c'],
    '<D>': ['d']
}

fuzzer = GrammarCoverageFuzzer(grammar, min_nonterminals=5, max_nonterminals=10)

for i in range(100):
    print(fuzzer.fuzz())
    print('-----------')