regexp: add type checks for `assert.Regexp` regexp argument

The signature for [`assert.Regexp`](https://pkg.go.dev/github.com/stretchr/testify/assert#Regexp) (and associated methods) is:

```go
func Regexp(t TestingT, rx interface{}, str interface{}, msgAndArgs ...interface{}) bool
```

However `rx` should only be `*regexp.Regexp` or `string`.

Related: stretchr/testify#1585
Cc: @kevinburkesegment
