fix typo in unit test on masking

resulting in only checking the first metric (i.e. BS) repeatedly

tests/test_probabilist.cpp

@@ -167,6 +167,6 @@ TEST(ProbabilistTests, TestMasks)
     // check results are identical
     for (int m = 0; m < metrics.size(); m++)
     {
-        EXPECT_TRUE(xt::allclose(metrics_masked[0], metrics_subset[0]));
+        EXPECT_TRUE(xt::allclose(metrics_masked[m], metrics_subset[m]));
     }
 }
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