tests: build against go1.15

Author: oboukili

.github/workflows/tests.yml

@@ -19,7 +19,7 @@ jobs:
       - uses: actions/checkout@v2
       - uses: actions/setup-go@v1
         with:
-          go-version: 1.14
+          go-version: 1.15
         id: go
       - name: Restore Go cache
         uses: actions/cache@v1