CI: run tests separately from build jobs

Reorganize GitHub Actions workflows to run tests the same way on all Go
versions, and so remove running tests from the 'build' jobs (which run
job only on 'oldstable' and 'stable').

This makes the build matrix of the 'test' job more consistent.

Author: dolmen

.github/workflows/main.yml

@@ -20,12 +20,14 @@ jobs:
       - run: ./.ci.gofmt.sh
       - run: ./.ci.readme.fmt.sh
       - run: ./.ci.govet.sh
-      - run: go test -v -race ./...
+
   test:
     runs-on: ubuntu-latest
     strategy:
       matrix:
         go_version:
+          - stable
+          - oldstable
           - "1.17"
           - "1.18"
           - "1.19"
@@ -40,4 +42,5 @@ jobs:
         uses: actions/setup-go@4a3601121dd01d1626a1e23e37211e3254c1c06c # v6.4.0
         with:
           go-version: ${{ matrix.go_version }}
-      - run: go test -v -race ./...
+      - name: Test
+        run: go test -v -race ./...