Is it possible to slice a string literal?

Can we slice a string literal like 'test'[0:3] in Go without loops in 1 line of code?

Yes.

Slice expressions apply to strings, including string literals:

For untyped string operands the result is a non-constant value of type string.

package main

import "fmt"

func main() {
    b := "Hello, 世界"[1:3]
    fmt.Println(b) 
    // prints el
}

The usual caveat is that slicing a string works with the string bytes. If you slice across multi-byte runes, you'll get corrupted data:

func main() {
    b := "Hello, 世界"[8:9]
    fmt.Println(b)
    // prints � and not 世
}