Bug in how standardUnitMultipliers is used in validator

[kerimoyle]

There's been a bug in how we test for multiplier mismatch where there are standardUnitMultipliers involved. This test below should return a warning because there are 1000L per m^3, but it was returning no mismatch. I'll put the super-quick fix in a new PR.

TEST(Validator, unitMultiplierFactorDifference)
{
    libcellml::ValidatorPtr validator = libcellml::Validator::create();
    libcellml::ModelPtr m = createModelTwoComponentsWithOneVariableEach("m", "c1", "c2", "v1", "v2");
    auto c1 = m->component(0);
    auto c2 = m->component(1);
    auto v1 = c1->variable(0);
    auto v2 = c2->variable(0);

    v1->setUnits("litre");

    libcellml::UnitsPtr u = libcellml::Units::create();
    u->setName("big_barrel");
    u->addUnit("metre", 3.0);
    m->addUnits(u);

    v2->setUnits(u);

    libcellml::Variable::addEquivalence(v1, v2); // litre ~= metre^3 // Nope: there are 1000L = 1m^3

    validator->validateModel(m);

    printIssues(validator);

    std::cout << "Issues: " << validator->issueCount() << std::endl;
    std::cout << "Errors: " << validator->errorCount() << std::endl;
    std::cout << "Warnings: " << validator->warningCount() << std::endl;
    std::cout << "Hints: " << validator->hintCount() << std::endl;

    EXPECT_EQ(size_t(1), validator->issueCount());  // << no issues reported, but there should have been
    EXPECT_EQ(size_t(0), validator->errorCount());
    EXPECT_EQ(size_t(1), validator->warningCount());

    EXPECT_EQ_ISSUES(expectedErrors, validator);
}