PhoneFieldDetailsTests: make constant static strings unowned

This works around Vala bug GNOME#646970, which is only fixed in a
development version (0.21). Newer Vala versions automatically make
these strings behave as if unowned anyway.

Bug: https://bugzilla.gnome.org/show_bug.cgi?id=706683
Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Reviewed-by: Philip Withnall <philip@tecnocode.co.uk>
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