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.

Also, added a new lines before each sleep command to better signal
the pause.
Compile-time constant dependency tracking moved to separate test.

The `final` modifier is not essential in `source-dependencies/export-jars`
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.

Testing for that scenario has been moved to separate test case
`source-dependencies/constants`.