Fix erratic NPE in LanguageServersTest#testCancellable

[sebthom]

This commit fixes the NPE:

org.eclipse.lsp4e.test.LanguageServersTest.testCancellable -- Time
elapsed: 0.112 s <<< ERROR!
java.lang.NullPointerException: Cannot invoke
"org.eclipse.jface.text.ITextViewer.getTextWidget()" because "viewer" is
null
	at org.eclipse.lsp4e.test.LanguageServersTest.testCancellable(LanguageServersTest.java:789)

which may occur for the code:

IEditorPart editor = TestUtils.openEditor(testFile);
ITextViewer viewer = LSPEclipseUtils.getTextViewer(editor);
IDocument document = viewer.getDocument();

where viewer may be null if it has not been initialized fast enough in the UI thread.

As a remedy, a waitForAndAssertCondition is added to the function openTextViewer:

public static ITextViewer openTextViewer(IFile file) throws PartInitException {
	IEditorPart editor = openEditor(file);
	waitForAndAssertCondition(5_000, () -> LSPEclipseUtils.getTextViewer(editor) != null);
	return LSPEclipseUtils.getTextViewer(editor);
}

and all occurrences of

IEditorPart editor = TestUtils.openEditor(testFile);
ITextViewer viewer = LSPEclipseUtils.getTextViewer(editor);

are replaced with

ITextViewer viewer = TestUtils.openTextViewer(testFile);

[sebthom]

Hmm...maybe the NPE is only thrown when an OOM occurred and a TextViewer instance could not be allocated.

[rubenporras]

thanks