.circleci: use a separate job for coverage testing

When using -race flag more memory can be consumed.

.circleci/config.yml

@@ -82,7 +82,19 @@ jobs:
       - run: git submodule sync
       - run: git submodule update --init
       - gomod
-      - run: go test -v -race ./... -coverprofile=coverage.txt -covermode=atomic -coverpkg=all
+      - run: go test -v -race ./...
+
+  test_cover:
+    working_directory: /go/src/github.com/nspcc-dev/neo-go
+    executor: go1_14
+    environment:
+        CGO_ENABLED: 0
+    steps:
+      - checkout
+      - run: git submodule sync
+      - run: git submodule update --init
+      - gomod
+      - run: go test -v ./... -coverprofile=coverage.txt -covermode=atomic -coverpkg=all
       - codecov/upload:
           file: coverage.txt
 
@@ -142,6 +154,10 @@ workflows:
           filters:
             tags:
               only: v/[0-9]+\.[0-9]+\.[0-9]+/
+      - test_cover:
+          filters:
+            tags:
+              only: v/[0-9]+\.[0-9]+\.[0-9]+/
       - build_cli:
           filters:
             tags: