Issue #583: drop reflection from XChartPanelSaveTargetTest

[timmolter]

Follow-up to the #583 Save As work. No behavior change — test cleanup only.

Problem

XChartPanelSaveTargetTest reached XChartPanel.SuffixSaveFilter reflectively:

Class<?> clazz = Class.forName("org.knowm.xchart.XChartPanel$SuffixSaveFilter");
Constructor<?> ctor = clazz.getDeclaredConstructor(String.class);
ctor.setAccessible(true);

Two things wrong with that:

• The class name is a string literal, so renaming or moving the class fails at runtime with ClassNotFoundException instead of at compile time.
• It's inconsistent within the same file. resolveSaveTarget and suffixOf are already package-private statics and the test calls them directly; only the filter was reached reflectively.

The constructor was already public — only the nested class was private, so setAccessible wasn't even the thing unblocking it.

Change

Relax SuffixSaveFilter to package-private and construct it directly:

private static FileFilter filter(String suffix) {
  return new XChartPanel.SuffixSaveFilter(suffix);
}

This matches the established idiom in the codebase — internal/chartpart has ~86 package-private members serving as test seams, and tests there read them directly (e.g. RegressionTestIssue577 asserts on legend.xOffset). SuffixSaveFilter stays invisible to library users; it was already nested inside XChartPanel with no public exposure.

Also drops the now-unneeded throws Exception from the five test methods and the java.lang.reflect.Constructor import.

Verification

Full xchart suite green (146 tests); XChartPanelSaveTargetTest itself runs 5/5. With this and #1009, no test in the tree uses reflection.

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