sanitycheck: check existence of testcase_extra.conf

Running sanitycheck for multiple platforms but only request coverage
on a single platform fails, example:
  sanitycheck --enable-coverage --coverage-platform nrf52840dk_nrf52840
  -p nrf52840dk_nrf52840 -p nrf52dk_nrf52832 -T <sample>

This happens because `testcase_extra.conf` will only be created if the
current platform is included in the list of coverage platforms.

The error in the example above that would be seen is:
  File not found:
      <...>/sanitycheck/testcase_extra.conf
This commit now uses the existence of `testcase_extra.conf` which is
created in the method `create_overlay()` before appending
the conf file to the list of overlay files.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>