memtx: add a couple of test cases to tx_man.test

That's strange, but in this test in a group of simple test cases
there are test cases that checks replaces, updates and deletes,
but occasionally there's no test case that checks inserts.

Fix it and add simple test cases for inserts.

No logical changes.

Part of #8648
Part of #8654

NO_DOC=new test case
NO_CHANGELOG=new test case