Fix build errors and failing tests

[Mr-Pine]

This fixes one clang compilation error in the MinimalScheduler sample and the two currently failing tests:

• Add @Unsigned annotation to division in MinimalScheduler sample
• Remove two extra newlines in CompilerPlugin.java, this previously caused TypeProcessingTest#testIncludesAndInterface to fail because of the extra newlines between x and y
• FEntryExitAutoAttachTest:
  • Change the used syscall to openat2 since this is what is triggered by TestUtil
  • Don't trace syscalls from other processes or with /sys/* files
  • Remove input.available() > 0 condition from LineReader.ready(). The trace pipe is not seekable, causing this condition to alway throw an exception, preventing LineReader from ever getting ready.
  • Change readAllAvailableLines to a do while-loop.

I'm not totally happy with the trace changes to FEntryExitAutoAttachTest, but the ready() behaviour of TraceLog/the underlying BufferedReader isn't great. For example, it often seems to require a blocking read() to report as ready. In some cases, this causes traces from one test to leak another one, which isn't ideal.
A solution with GlobalVariables or a ring buffer would probably be better (for all tests).

[parttimenerd]

It would probably be best.