Document why sleeps are needed in `export-jars` test.

Git blaming reveals the intent of sleeping after file modification
that was described in 02f666ec0eafa48498fe850e76c5e77d386c99bd commit.
Let's put it straight into source code.
Remove `final` modifiers in `export-jars` test case.

The modifier is not essential and removing it eliminates the danger of
running into Scala's bug SI-7173 where typer inlines constants and does
not record any information about dependency. In case we want to switch
to dependency tracking based on trees we would run into this problem in
this test-case.