Bumping gaia.json for 1 gaia revision(s) a=gaia-bump

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https://hg.mozilla.org/integration/gaia-central/rev/e4c15ca33e7f
Author: Anthony Ricaud <anthony@ricaud.me>
Desc: Bug 1034279 - USSD messages not displayed if user leaves Dialer while sending r=etienne

I took the opportunity to re-enable a unit test
Bumping manifests a=b2g-bump