test: use a non system PCR for testing PCR extend

We currently use PCR 0 for testing the PCR read/extend functionality in
our selftests.  How ever those PCRs are defined by the TCG spec for
platform use.  For example if the tests run *after* the efi subsystem
initialization, which extends PCRs 0 & 7 it will give a false positive.

So let's switch over to a PCR which is more suitable and is defined for
OS use.  It's worth noting that we are using PCR10 here, since PCR9 is
used internally by U-Boot if we choose to measure the loaded DTB

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>