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This CL adds test coverage for the appearance of the Expandable
Payment Handler UI and its user interaction.

Changes:
* Moved the original content of ExpandablePaymentHandlerTest.java into
ExpandablePaymentHandlerCoordinatorTest.java (and change to small-test).
* Added test cases in ExpandablePaymentHandlerTest.java.
* Changed payment method from "/pay" to the "payment_method.json".
This is because /pay was not pointing to a payment method manifest, so
JIT installation was disabled.
* merchant.html: given ids to the buttons so that the tests can
simulate users' clicking on the buttons.

Bug: 1042892, 1084705, 1084707, 1084708, 1084711, 1084721, 1068741

Change-Id: I4943932632cc446c75144bcdf7f9d9f9e06881be
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2227512Reviewed-by: Rouslan Solomakhin <rouslan@chromium.org>
Commit-Queue: Liquan (Max) Gu <maxlg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#775203}