Most of the sample tests are incorrect.

assertArrayEquals(new String[][]{
          {"2", "+", "3", "=", "5"},
          {"+", " ", "+", " ", "+"},
          {"4", "+", "1", "=", "5"},
          {"+", " ", "+", " ", "+"},
          {"6", "+", "2", "=", "8"} // 5 + 5 != 8, the symbols in the fourth row should = instead of +.
      }, CrossMath.solve(new String[][]{
          {"2", "+", "3", "=", "C"},
          {"+", " ", "+", " ", "+"},
          {"A", "+", "1", "=", "5"},
          {"+", " ", "+", " ", "+"},
          {"6", "+", "B", "=", "8"}
      }));

assertArrayEquals(new String[][]{
          {"1", "+", "3", "=", "4"},
          {"+", " ", "-", " ", "+"},
          {"3", "-", "3", "=", "0"},
          {"=", " ", "=", " ", "="},
          {"4", "+", "0", "=", "8"} // 4 + 0 != 8.
      }, CrossMath.solve(new String[][]{
          {"1", "+", "3", "=", "4"},
          {"+", " ", "-", " ", "+"},
          {"A", "-", "3", "=", "0"},
          {"=", " ", "=", " ", "="},
          {"B", "+", "C", "=", "8"}
      }));

assertArrayEquals(new String[][]{
          {"48", "/", "2", "=", "24"},
          {"/",  " ", "/",  " ", "/"},
          {"24", "/", "2", "=", "12"},
          {"=",  " ", "=",  " ", "="},
          {"12", "/", "2", "=", "6"} // 48 / 24 != 12; 2 / 2 != 2; 24 / 12 != 6.
      }, CrossMath.solve(new String[][]{
          {"48", "/", "C", "=", "24"},
          {"/",  " ", "/",  " ", "/"},
          {"24", "/", "B", "=", "12"},
          {"=",  " ", "=",  " ", "="},
          {"A", "/", "2", "=", "6"}
      }));

Duplicate: https://www.codewars.com/kata/5aba0a08379d20026e0000be

src/test/java/CraftingCheckerTest.java:9: error: cannot find symbol
    private static final String STONE = CraftingChecker.STONE;
                                                       ^
  symbol:   variable STONE
  location: class CraftingChecker
src/test/java/CraftingCheckerTest.java:10: error: cannot find symbol
    private static final String STICK = CraftingChecker.STICK;
                                                       ^
  symbol:   variable STICK
  location: class CraftingChecker
2 errors

I think it would make more sense for the tests to stand alone and not depend on static constants in the solution.