zephyr: tests: Fix flaky BMI260 tests
zephyr: tests: Fix flaky BMI260 tests

Add a before function and fix up some setup bugs in BMI260 unit tests.
With these fixes, I was able to run the test executable 20 times with
random seeds and not hit any BMI260 errors.

BRANCH=None
BUG=b:233100454,b:233099844,b:233099880
TEST=util/shuffle_test.sh "TESTSUITE bmi260 failed"

Signed-off-by: Tristan Honscheid <1e17951ff5ffc8afe254954c03de64afef62fe52@google.com>
Change-Id: I77db0682c467424033d7984240db13ae7b3d9fba
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/3761060
Reviewed-by: Jeremy Bettis <4df7b5147fee087dca33c181f288ee7dbf56e022@chromium.org>

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