Use indoc for JSON document literals in test

[dtolnay]

Indoc takes a string literal and un-indents it so the leftmost non-space character is in the first column. Let me know whether you think this is valuable.

Before:

concat!(
    "[\n",
    "  false,\n",
    "  null,\n",
    "  [\n",
    "    \"foo\\nbar\",\n",
    "    3.5\n",
    "  ]\n",
    "]"
)

After:

indoc!(r#"
    [
      false,
      null,
      [
        "foo\nbar",
        3.5
      ]
    ]"#
)

[erickt]

Neat! That does look a lot cleaner now!