Merge pull request #15810 from eileenmcnaughton/mem_fix

[REF] calculate 'amount' on ContributionPage in a shared way in one scenario

diff --cc CRM/Contribute/Form/Contribution/Confirm.php
Simple merge

diff --cc CRM/Contribute/Form/Contribution/Main.php
Simple merge

diff --cc CRM/Contribute/Form/ContributionBase.php
Simple merge

diff --cc tests/phpunit/api/v3/ContributionPageTest.php
Simple merge