test: run Test workflow on v1.19 branch (#5140)

**Which issue(s) this PR fixes**: 
Fixes #

**What this PR does / why we need it**: 
When we create the PR for v1.19 branch, now, Test workflow does not
work.
This patch will fix to work Test workflow.

**Docs Changes**:

**Release Note**:

Signed-off-by: Shizuo Fujita <fujita@clear-code.com>

Author: Watson1978

.github/workflows/test.yml

@@ -2,12 +2,12 @@ name: Test
 
 on:
   push:
-    branches: [master]
+    branches: [v1.19]
     paths-ignore:
       - '*.md'
       - 'lib/fluent/version.rb'
   pull_request:
-    branches: [master]
+    branches: [v1.19]
     paths-ignore:
       - '*.md'
       - 'lib/fluent/version.rb'