Refactor the startup logic in fflush.t to use skip_all()

Refactor the startup logic in fflush.t to use skip_all()

The tautological 3 way logic
    if (...) { print "1..7\n" } elsif (...) { print "1..7\n" } else { ... }
had actually been in the test since it was first added in a43cb6b7285a7b60,
although it was initially "1..4\n" in each block.

Also use plan(), and set test.pl's idea of the current test before exiting.