Test: Increase the UART flush delay

[fkjagodzinski]

Summary of changes

• Increase the value of SERIAL_FLUSH_TIME_MS used to wait for the end of the greentea communication in a couple of tests. The original calculation was based on the 16-byte UART Tx FIFO assumption. This is wrong for CY8CPROTO_062_4343W which uses 128-byte FIFO.
• Insert UART-flush wait before watchdog reset.

This patch is based on the findings from #11909 (comment).

Fixes #11909

Impact of changes

Migration actions required

Documentation

None

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Pull request type

[x] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

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Test results

[] No Tests required for this change (E.g docs only update)
[x] Covered by existing mbed-os tests (Greentea or Unittest)
[] Tests / results supplied as part of this PR

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Reviewers

@jamesbeyond, @dustin-crossman

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[ciarmcom]

@fkjagodzinski, thank you for your changes.
@jamesbeyond @ARMmbed/mbed-os-core @ARMmbed/mbed-os-test @ARMmbed/mbed-os-hal @ARMmbed/mbed-os-maintainers please review.

[jamesbeyond]

looks good to me, thanks for the fixing!

[0xc0170]

Ci started

[mbed-ci]

Test run: SUCCESS

Summary: 5 of 5 test jobs passed
Build number : 1
Build artifacts

[0xc0170]

Also the SERIAL_FLUSH_TIME_MS comment mentions that there is currently no mbed API to check if hardware buffers are empty. Do you know if there is any progress on that front? It seems like it would help a lot to prevent similar bugs on other hardware/tests.

From the issue referenced, 👍 having this, to avoid doing this in tests/apps.

[0xc0170]

My above comment: @fkjagodzinski Do you know where is this being tracked ?