Automatically clear diagnostics in test fixture

As per the LSP spec, the client is not responsible for merging
diagnostics, therefore our publishDiagnostics test implementation should
honor this.

See https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#textDocument_publishDiagnostics

Author: fwcd

server/src/test/kotlin/org/javacs/kt/DiagnosticTest.kt

@@ -21,7 +21,6 @@ class DiagnosticTest : SingleFileTestFixture("diagnostic", "Diagnostics.kt") {
 
     @Test fun `report only errors`() {
         languageServer.config.diagnostics.level = DiagnosticSeverity.Error
-        diagnostics.clear()
 
         // Trigger a diagnostics update via a dummy change.
         // TODO: Use the LSP configuration change notification mechanism instead
@@ -54,7 +53,6 @@ class DiagnosticTest : SingleFileTestFixture("diagnostic", "Diagnostics.kt") {
         languageServer.textDocumentService.debounceLint.waitForPendingTask()
         languageServer.textDocumentService.lintRecompilationCallback = {
             if (callbackCount++ == 0) {
-                diagnostics.clear()
                 replace(file, 7, 9, "return 11", "")
                 languageServer.textDocumentService.documentSymbol(DocumentSymbolParams(TextDocumentIdentifier(uri(file).toString()))).get()
             }

server/src/test/kotlin/org/javacs/kt/LanguageServerTestFixture.kt

@@ -151,6 +151,7 @@ abstract class LanguageServerTestFixture(relativeWorkspaceRoot: String) : Langua
     // LanguageClient functions
 
     override fun publishDiagnostics(diagnostics: PublishDiagnosticsParams) {
+        this.diagnostics.clear()
         this.diagnostics.addAll(diagnostics.diagnostics)
     }