Cleanup tests - using assert method instead of assert keyword
Remove `omit_term_freq_and_positions` for new indices

`omit_term_freq_and_positions` was deprecated in `0.20` and
is not documented anymore. We should reject indices that are
created with this option in the future.

Closes #4722
Add SecurityManger / policy when running tests.

This commit adds a security manager to the test JVMs
that prevents mainly writing files outside of the JVMs
current test directory.
[SPEC] Added missing body object in snapshot apis
Use millis for joda DateTimeFormatter.print()

Fixes #4782.
[SPEC] removing deprecated include/exclude for get_source

_source_include/_source_exclude should be used instead
Removed double masterNodeTimeout set in RestPutRepositoryAction
[TEST] fixed error message in REST tests runner
Check ThreadInfo[] for null element if thread are not alive.

If a thread is not alive getting ThreadMXBean#getThreadInfo(long[], int)
places null elemnents in the returned array which are not repected
in the HotTheards API.

Closes #4775
[SPEC] Removed _source endpoint from get api spec, as the get_source is a different api already

Also removed custom code that was required in the REST tests due to this.