tools/mpremote: Support trailing slash on dest for non-recursive copy.

[dpgeorge]

Summary

This fixes a regression in db59e55: prior to that commit mpremote supported trailing slashes on the destination of a normal (non-recursive) copy.

Add back support for that, with the semantics that a trailing slash requires the destination to be an existing directory.

Testing

A test has been added to the mpremote tests (also a test for force copy, in a separate commit). As part of getting the test working, needed to flush stdout before printing error messages to stderr.

The mpremote tests were then run on a PYBD_SF2. They passed.

[dpgeorge]

Note: this is related to #2466 and #2929: filesystems on bare-metal ports don't like trailing slashes in their filenames, even if it refers to a directory. So that's why trailing slashes need to be stripped before passing them down to the target board (also doing the strip in mpremote allows enforcing that anything ending in a slash is indeed a directory).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.57%. Comparing base (6902362) to head (4fd5b72).
Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #16164   +/-   ##
=======================================
  Coverage   98.57%   98.57%           
=======================================
  Files         164      164           
  Lines       21345    21345           
=======================================
  Hits        21041    21041           
  Misses        304      304           

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[github-actions]

Code size report:

   bare-arm:    +0 +0.000% 
minimal x86:    +0 +0.000% 
   unix x64:    +0 +0.000% standard
      stm32:    +0 +0.000% PYBV10
     mimxrt:    +0 +0.000% TEENSY40
        rp2:    +0 +0.000% RPI_PICO_W
       samd:    +0 +0.000% ADAFRUIT_ITSYBITSY_M4_EXPRESS
  qemu rv32:    +0 +0.000% VIRT_RV32

[dpgeorge]

This would also go in a v1.24.1 patch release.

[projectgus]

LGTM!

[dpgeorge]

For some reason the windows mingw dev CI is failing, but that's also happening on master so it's nothing to do with this PR. That needs separate investigation.