“Crucible” can mean a test or a container used to heat up chemicals to high temperatures in order to reach a melting point or create a chemical reaction.
In terms of a test, this was a test of the moral judgment of the town as a whole. They failed this test with flying colors. There is also a secondary satirical meaning on test. The trials themselves test the guilt or innocence of the accused. These tests are biased and flawed because they are controlled by religious fundamentalism and conformity.
As the hysteria increases, the accusations accumulate and Salem reaches a melting point where their identity changes. They define themselves by witch hunting more than religious belief. The parallel is between heating chemicals to a crucial degree and the rising hysteria of the mob mentality reaching a point of no return.