add support for multi-file test suite

When collecting tests or evaluating whether a symbol is a test for CodeLens, check if testify was imported by any file of the directory (which is the go package).

Not sure whether this is the optimal / recommended way of dealing with this, but here goes.

Fixes #1130

Change-Id: I4e66f0d319604a7300116240c24b60dcdd37bf81
GitHub-Last-Rev: efc72ad
GitHub-Pull-Request: #2314
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/415894
TryBot-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
Run-TryBot: Hyang-Ah Hana Kim <hyangah@gmail.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

Author: nirhaas

src/goDocumentSymbols.ts

@@ -102,5 +102,5 @@ async function listImports(
 		]
 	};
 	const resp = await languageClient?.sendRequest(ExecuteCommandRequest.type, params);
-	return resp.Imports;
+	return resp.PackageImports;
 }

src/testUtils.ts

@@ -158,6 +158,9 @@ export async function getTestFunctions(
 		return;
 	}
 	const children = symbol.children;
+
+	// With gopls dymbol provider symbols, the symbols have the imports of all
+	// the package, so suite tests from all files will be found.
 	const testify = importsTestify(symbols);
 	return children.filter(
 		(sym) =>