Adapt lcov_coverage_test to exec.Command change (bazelbuild#3392)

`exec.Command` no longer implicitly looks in the current directory with Go 1.19.

tests/core/coverage/lcov_coverage_test.go

@@ -131,7 +131,7 @@ func TestLib(t *testing.T) {
 }
 
 func TestTool(t *testing.T) {
-	err := exec.Command("." + string(filepath.Separator) + "Tool").Run()
+	err := exec.Command(filepath.Join(".", "Tool")).Run()
 	if err != nil {
 		t.Error(err)
 	}