expect_snapshot() fails when names contain backquotes

When running such test:

```
test_that("backquotes", {
  expect_snapshot(
    c("`a`" = 1)
  )
})
```

I get : 

```
`c(`\`a\`` = 2)` threw an unexpected error.
Message: attempt to use zero-length variable name
Class:   simpleError/error/condition
```