twister: Default platforms
twister: Default platforms

Default to running tests for native_posix and unit_testing if the user
doesn't specify a platform.

BRANCH=None
BUG=None
TEST=./twister -v -i

Signed-off-by: Jeremy Bettis <4df7b5147fee087dca33c181f288ee7dbf56e022@google.com>
Change-Id: Ibf0da0917ca2b78d46d55638c25fca2b2cc5ba40
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/3833912
Reviewed-by: Tristan Honscheid <1e17951ff5ffc8afe254954c03de64afef62fe52@google.com>
Auto-Submit: Jeremy Bettis <4df7b5147fee087dca33c181f288ee7dbf56e022@chromium.org>
Commit-Queue: Jeremy Bettis <4df7b5147fee087dca33c181f288ee7dbf56e022@chromium.org>
Tested-by: Jeremy Bettis <4df7b5147fee087dca33c181f288ee7dbf56e022@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec