Update GHA CI workflow

- Enable GHA workflow for Pull Requests so that required checks work
- Remove `cargo-tarpaulin` as we aren't using the output of the code
  coverage check, and the action-rs project appears to be unmaintained
  and non-functional.
- Target a specific ref in the `http-signatures-test-suite` repo as the
  branch tag has gone missing.

.github/workflows/toolchain.yml

@@ -1,4 +1,4 @@
-on: [push]
+on: [push, pull_request]
 
 name: CI
 
@@ -63,23 +63,12 @@ jobs:
           toolchain: stable
           override: true
 
-      - name: Run cargo-tarpaulin
-        uses: actions-rs/[email protected]
+      - name: Run cargo test
+        uses: actions-rs/cargo@v1
         with:
-          version: latest
+          command: test
           args: --all --all-features
 
-      - name: Upload to codecov.io
-        uses: codecov/[email protected]
-        with:
-          token: ${{secrets.CODECOV_TOKEN}}
-
-      - name: Archive code coverage results
-        uses: actions/upload-artifact@v1
-        with:
-          name: code-coverage-report
-          path: cobertura.xml
-
   rfc-test-suite:
     name: RFC Test Suite
     runs-on: ubuntu-latest
@@ -92,7 +81,7 @@ jobs:
         with:
           repository: Diggsey/http-signatures-test-suite
           path: http-signatures-test-suite
-          ref: remove-echo
+          ref: 872fad28a85d2f87f042b55ed05ca7a1d43d977b
 
       - name: Setup Node.js
         uses: actions/setup-node@v1