[chore] clean up reexec.Init() calls

Now that most uses of reexec have been replaced with non-reexec
solutions, most of the reexec.Init() calls peppered throughout the test
suites are unnecessary. Furthermore, most of the reexec.Init() calls in
test code neglects to check the return value to determine whether to
exit, which would result in the reexec'ed subprocesses proceeding to run
the tests, which would reexec another subprocess which would proceed to
run the tests, recursively. (That would explain why every reexec
callback used to unconditionally call os.Exit() instead of returning...)

Remove unneeded reexec.Init() calls from test and example code which no
longer needs it, and fix the reexec.Init() calls which are not inert to
exit after a reexec callback is invoked.

Signed-off-by: Cory Snider <csnider@mirantis.com>

Signed-off-by: Jeyanthinath Muthuram <jeyanthinath10@gmail.com>

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

Implement a ReadJSON() utility to help reduce some code-duplication,
and to make sure we handle JSON requests consistently (e.g. always
check for the content-type).

Differences compared to current handling:

- prevent possible panic if request.Body is nil ("should never happen")
- always require Content-Type to be "application/json"
- be stricter about additional content after JSON (previously ignored)
- but, allow the body to be empty (an empty body is not invalid);
  update TestContainerInvalidJSON accordingly, which was testing the
  wrong expectation.
- close body after reading (some code did this)

We should consider to add a "max body size" on this function, similar to
https://github.com/moby/moby/blob/7b9275c0da707b030e62c96b679a976f31f929d3/api/server/middleware/debug.go#L27-L40

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

- Bring mediaType out of reserved status
- specs-go: adding mediaType to the index and manifest structures

full diff: https://github.com/opencontainers/image-spec/compare/v1.0.1...v1.0.2

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

The io/ioutil package has been deprecated in Go 1.16. This commit
replaces the existing io/ioutil functions with their new definitions in
io and os packages.

Signed-off-by: Eng Zer Jun <engzerjun@gmail.com>

Discovered a few instances, where loop variable is incorrectly used
within a test closure, which is marked as parallel.
Few of these were actually loops over singleton slices, therefore the issue
might not have surfaced there (yet), but it is good to fix there as
well, as this is an incorrect pattern used across different tests.

Signed-off-by: Roman Volosatovs <roman.volosatovs@docker.com>

This was changed as part of a refactor to use containerd dist code. The
problem is the OCI media types are not compatible with older versions of
Docker.

Signed-off-by: Brian Goff <cpuguy83@gmail.com>