Mongod servers started through the shell do not respect TestData.enableMajorityReadConcern

We only add enableMajorityReadConcern to the command line arguments if it is set to true in the TestData. This means that mongods started test fixtures do not respect TestData.enableMajorityReadConcern="false". Passthroughs are unaffected. Instead of checking whether jsTest.options().enableMajorityReadConcern is true, we should check that it is not equal to undefined. We should also add a test to ensure that if we set TestData.enableMajorityReadConcern, then the server reports that it does not support committed reads.