fix: Allow test and cover script to run without enabling Rust coverage (

#126)

Sometimes, we want to run only specific tests that don't include the NSS
module (which means no Rust coverage). The script quit with an error if
it could not merge the Rust coverage to the Go one. This is fine for the
CI since we want all coverage, but the script needs a change to prevent
this.

gotestcov

@@ -46,7 +46,9 @@ go test -cover -covermode=set $@ -shuffle=on -args -test.gocoverdir="${raw_cov_d
 go tool covdata textfmt -i="${raw_cov_dir}" -o="${cov_dir}/coverage.out"
 
 # Append the Rust coverage data to the Go one
-cat "${raw_cov_dir}/rust-cov/rust2go_coverage" >>"${cov_dir}/coverage.out"
+if [ -f "${raw_cov_dir}/rust-cov/rust2go_coverage" ]; then
+    cat "${raw_cov_dir}/rust-cov/rust2go_coverage" >>"${cov_dir}/coverage.out"
+fi
 
 # Filter out the testutils package and the pb.go file
 grep -v -e "testutils" -e "pb.go" "${cov_dir}/coverage.out" >"${cov_dir}/coverage.out.filtered"