test/gopls: actually flush the gopls trace after each test

Otherwise, the trace may grow too big to be printed in github CI

Change-Id: I0ed9051d84ce268d390bd68bdca9bb38fd1f9d4a
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/268878
Trust: Hyang-Ah Hana Kim <hyangah@gmail.com>
Run-TryBot: Hyang-Ah Hana Kim <hyangah@gmail.com>
TryBot-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Suzy Mueller <suzmue@golang.org>
diff --git a/test/gopls/extension.test.ts b/test/gopls/extension.test.ts
index 7e82399..5d4d93c 100644
--- a/test/gopls/extension.test.ts
+++ b/test/gopls/extension.test.ts
@@ -63,6 +63,7 @@
 		if (print) {
 			console.log(this.fakeOutputChannel.toString());
 		}
+		this.fakeOutputChannel.clear();
 	}
 
 	// This is a hack to check the progress of package loading.