Scripts: Add script tag attribute handling tests

[sirreal]

Add tests that verify the behavior of wp_get_inline_script_tag() wp_get_script_tag() attribute handling. This is relevant for the work in #10639. See #10639 (comment).

Tests are added using assertEqualHTML() assertions that verify the behavior when:

• Different attribute values and types are provided.
• Repeated attribute names are provided (correct order handling)

Trac ticket: https://core.trac.wordpress.org/ticket/64500

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[dmsnell]

I have always wondered if we should treat null and false values equivalently in set_attribute(). This scenario almost makes it seem important to do so, because who is going to expect the string value "null" to appear when sending a null value?

[dmsnell]

This is a nice touch in preparation for #10639

[sirreal]

Merged in r61482.