Update known buildbot failures for ProcessBuilderTest

Update known buildbot failures for ProcessBuilderTest

Also updated the comment string based on our latest understanding.
This is a native crash. Both of the affected tests as well as the
other tests in ProcessBuilderTest run 'sh' as a child process.
Only one of the tests that are affected to far accesses /proc/,
which had previously been suspected as related to the cause.

The statement that this is a native crash is based on there not
being a relevant stacktrace in logcat.

A 'sh' related native crash failure occurs in the logs but that
is not specific to these tests so it's not clear if it is related.
If not, it's not clear why the additional native crash doesn't
show up in the logs.

Test: Nothing

Bug: 30657148

Change-Id: I8347edab3cacd060baf1087a1939926f70fdd349