refactor: add test file

data/tetests/spsolve_test.txt

+g = [x1+3/3 * x2+ min(2,5)*x3 - 1 = 0, 1*3*x2 - 4*3 = 0, 1*3 * x2+sin(8-2)*x3 - 7 = 0];
+r = sp_linsolvex(g, [x1,x2,x3]);
+return r;
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