check the number of atom on amino acid, before and after adding modification

tests/test_aa.cpp

@@ -135,8 +135,17 @@ TEST_CASE("Amino Acid test suite.", "[Aa]")
 
     Aa alaninebis('A');
 
+    REQUIRE(alaninebis.getNumberOfAtom(AtomIsotopeSurvey::C) == 3);
+    REQUIRE(alaninebis.getNumberOfAtom(AtomIsotopeSurvey::H) == 5);
+
+    // xref: DiffFormula: "C 2 H 3 N 1 O 1"
     alaninebis.addAaModification(AaModification::getInstance("MOD:00397"));
 
+    REQUIRE(alaninebis.getNumberOfAtom(AtomIsotopeSurvey::C) == 5);
+    REQUIRE(alaninebis.getNumberOfAtom(AtomIsotopeSurvey::H) == 8);
+
+    REQUIRE(alaninebis.toAbsoluteString().toStdString() == "A(MOD:00397)");
+
     alaninebis.addAaModification(
       AaModification::getInstanceCustomizedMod(18.022316354654));
     // SUCCESS