Improve Filesystem Conformance tests

[dgerd]

In what area(s)?

/area API
/area autoscale
/area build
/area monitoring
/area networking
/area test-and-release

Describe the feature

Our filesystem conformance tests currently check for existence of files and mode bits where required. However, this check is not necessarily sufficient to ensure that the file is accessible to all compliant containers. We should extend this test to actually attempt to read/write files (where applicable) to ensure that nothing else is blocking access.

Related to #4074 (review)

[markusthoemmes]

Non-root is solved meanwhile I think. Reading and writing is missing here.

[dgerd]

Updated original issue. It should be accurate now. You are correct that actually reading and writing in addition to checking mode bits is the improvement here.

[dgerd]

Moving this out of Serving 0.8 and into Ice Box. This is low priority and has not been picked up.

[shashwathi]

/assign @andrew-su
/assign @shashwathi