[SUREFIRE-2009] Refactoring of surefire-junit3. JUnitTestSetExecutor and PojoTestSetExecutor should be stateless.

[Tibor17]

This is a refactoring of surefire-junit3 I needed to do before completing SUREFIRE-1860. The changes in SUREFIRE-1860 are big and therefore I would like to split them to this refactoring, abstraction in another PR, continue with another PRs regarding implementation of encoder/decoder, SimpleReportEntry. It would give us the opportunity to associate the std/out/err logs with test run id (Thread) and deterministically create the reports and this way fix pending issues (junit5, and simplify the listeners in junit4.7 provider). So I am splitting the work in several pieces.

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[slawekjaranowski]

JUnit3Provider - filed private TestsToRun testsToRun can be local value.
used only in JUnit3Provider#invoke

in JUnit3Provider#getSuites can be inline