test: fix box/transaction.test.lua

The ; separator was missed after select (check the first hunk) and as
a result the rollback was not executed. So the next test was failed merely
due to the unfinished transaction of the previous test.

We also need to fix the third test to use a different tuple.
Also fix a couple of comments.

NO_DOC=test fix
NO_TEST=test fix
NO_CHANGELOG=test fix