github: Bump up Go version in testing workflow (#4051)

diff --git a/.github/workflows/testing.yml b/.github/workflows/testing.yml
index 9784f9e..15d6311 100644
--- a/.github/workflows/testing.yml
+++ b/.github/workflows/testing.yml
@@ -20,7 +20,7 @@
       - name: Setup Go
         uses: actions/setup-go@v2
         with:
-          go-version: 1.14
+          go-version: 1.15
       - name: Checkout repo
         uses: actions/checkout@v2
 
@@ -38,20 +38,20 @@
       matrix:
         include:
           - type: vet
-            goversion: 1.14
+            goversion: 1.15
           - type: race
-            goversion: 1.14
+            goversion: 1.15
           - type: 386
-            goversion: 1.14
+            goversion: 1.15
           - type: retry
-            goversion: 1.14
+            goversion: 1.15
           - type: extras
+            goversion: 1.15
+          - type: tests
             goversion: 1.14
           - type: tests
             goversion: 1.13
           - type: tests
-            goversion: 1.12
-          - type: tests
             goversion: 1.11  # Keep until interop tests no longer require Go1.11
 
     steps: