freertos-libs-tests: Fix closefile-validsignature OTA Pal test

The fixes are provided with a patch that does the following:
* Make the binary file path configurable using
  a macro (`OTA_PAL_FIRMWARE_FILE`) instead of
  hardcoding it as `test_happy_path_image.bin`.
  This is so it can be specified as expected by the
  AWS test suite.
* Replace the RSA SHA256 signature as it is invalid.
  The replacement signature has been regenerated using
  a different key pair with the public key also added
  to the `src/ota/test_files` subdirectory.

Signed-off-by: Ahmed Ismail <Ahmed.Ismail@arm.com>