Fix Greentea test code with wakeup from deepsleep

[ccli8]

Description

This PR is to fix the same issue as #7224. Still take NUMAKER_PFM_NUC472 as an example: Its UART peripheral has 16-byte Tx FIFO. With baud rate set to 9600, flush Tx FIFO would take: 16 * 8 * 1000 / 9600 = 13.3 (ms). So set wait time to 20ms here for safe.

Pull request type

[X] Fix
[ ] Refactor
[ ] New target
[ ] Feature
[ ] Breaking change

[0xc0170]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 2410
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/7288/

Triggering tests

/morph test
/morph uvisor-test
/morph export-build
/morph mbed2-build

[mbed-ci]

Exporter Build : SUCCESS

Build number : 2043
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/7288/

[mbed-ci]

Test : SUCCESS

Build number : 2194
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/7288/2194