Include coverage from downstream tests (#48)

Similar to JuliaApproximation/ApproxFunBase.jl#199

Author: jishnub

.github/workflows/downstream.yml

@@ -41,7 +41,7 @@ jobs:
             # force it to use this PR's version of the package
             Pkg.develop(PackageSpec(path="."))  # resolver may fail with main deps
             Pkg.update()
-            Pkg.test()  # resolver may fail with test time deps
+            Pkg.test(; coverage = true)  # resolver may fail with test time deps
           catch err
             err isa Pkg.Resolve.ResolverError || rethrow()
             # If we can't resolve that means this is incompatible by SemVer and this is fine
@@ -50,3 +50,7 @@ jobs:
             @info "Not compatible with this release. No problem." exception=err
             exit(0)  # Exit immediately, as a success
           end
+      - uses: julia-actions/julia-processcoverage@v1
+      - uses: codecov/codecov-action@v2
+        with:
+          files: lcov.info