quoted: applying TestComments in tests

[manuelarte]

It seems there is a bit of inconsistency in the formatting/output of the failure messages of the tests.
As an example, tests like directive_test.go are using the failure messages as described in https://go.dev/wiki/TestComments#got-before-want.
But some other tests, like the ones in quoted_test.go are using the (not recommended) approach of:
"got %v, want %v"

This PR tries to follow the failure message recommendation described in https://go.dev/wiki/TestComments:

• Adding the tested function at the beginning.
• Changing the input parameter name to in.

So, as an example, this is a before/after for the two tests I changed:

Split:

• Before: "got ["a"]; want ["a "]"
• After: "Split("a") = ["a"], want ["a "]"

Join:

• "got a; want ab"
• "Join([a]) = a, want ab"

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