Add -testQuiet to report only failing tests

[Donach]

Motivation

On a project with hundreds of @test functions, wurst -runtests prints two or more
lines per passing test plus everything the tests themselves print. In CI that buries the
handful of lines that actually matter, even though the summary at the end already reports
what passed.

-compactOutput (#1188) shortens the output, but it does so by dropping detail for
failures too: no assertion message, no stack trace, no output from the failing test.

Change

Adds -testQuiet (short form -tq), which reports only what is wrong:

• Output of a passing test is collected and dropped once the test passes.
• A failing test prints exactly what it prints today: its own output, the assertion
  message, the stack trace and, for timeouts, the stack frames.
• The summary is always printed.

Composes with -compactOutput; neither flag changes the behavior of the other.

Fixes to the interpreter output redirection

RunTests.redirectInterpreterOutput was dead code, so output the tests themselves printed
never reached RunTests and went straight to System.err. Three separate reasons, all
fixed here, because quiet mode can only suppress output that it actually receives:

• ProgramStateIO shadowed ProgramState.outStream with its own field and overrode
  getOutStream/setOutStream without forwarding to the native providers. Removed; the
  base class already implements both and does the forwarding.
• ReflectionNativeProvider.setOutStream was an empty method, so OutputProvider (which
  implements println, BJDebugMsg, DisplayTextToPlayer, ...) kept writing to
  its default System.err. It now forwards to the OutputProvider it owns.
• The PrintStream wrapping the redirect had no autoflush, so anything short of 8 KB
  stayed in its buffer forever. Now autoflush with an explicit UTF-8 charset.

The visible consequence is that output printed by tests during -runtests now goes to the
stream RunTests prints to (stdout on the CLI) instead of System.err, which is what the
redirect was written to do. It also makes -compactOutput suppress that output, as
intended.

Tests

RunTestsQuietTests compiles a package with one passing and one failing test and asserts
on the produced output: quiet mode hides the passing test and its output, keeps the failing
test's output and assertion message, and keeps the summary; the default mode still reports
passing tests. Flag parsing is covered for both -testQuiet and -tq.

[Donach]

Closing in favour of #1205.

The -testQuiet flag here is redundant: Grill's --quiet already covers this use case (it passes -compactOutput and filters the output, keeping failure headers and Test failed: lines), which is the layer our CI actually invokes.

What is worth keeping are the three bugs I ran into while testing the flag, all of which make RunTests.redirectInterpreterOutput dead code: ProgramStateIO shadowing ProgramState.outStream, the empty ReflectionNativeProvider.setOutStream, and the missing autoflush on the PrintStream. #1205 contains only those, with a regression test.