[test][zxdb] Prevent rogue fx test debugger spawn.

Specify `--use-existing-debugger` to prevent `fx test` from spawning its
own `ffx debug connect` instance when `--break-on-failure` is specified,
preventing any rogue target-side `debug_agent` processes which can cause
flakes when debugging tests.

Depends-On: Ia35dc06c163063e7c7d40ffc5e7e0ce164d46ef0
Fixed: 443064881
Change-Id: Ifaadf7a2df0e4e2021df5332ce269c59bad8fb14
Reviewed-on: https://fuchsia-review.googlesource.com/c/vscode-plugins/+/1365591
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Amy Hu <amyhu@google.com>
Dependencies-Satisfied: CL Deps Checker <cl-deps-checker@fuchsia-internal-service-accts.iam.gserviceaccount.com>

src/test_controller.ts

diff --git a/src/test_controller.ts b/src/test_controller.ts
index de42dc9..5b4ac20 100644
--- a/src/test_controller.ts
+++ b/src/test_controller.ts
@@ -417,7 +417,7 @@
       }
 
       if (debug) {
-        args.push('--break-on-failure');
+        args.push('--break-on-failure', '--use-existing-debugger');
         const hasAttached = await vscode.commands.executeCommand('fuchsia.zxdb.attach', testURL);
         if (!hasAttached) {
           const message = new vscode.TestMessage(