Fix example(local=TRUE) for cedta (again)

[MichaelChirico]

Split off from #7833 to keep the purposes separate: here, a bugfix with NEWS entry, there, test-only edits to get the suite more robust to different platforms/R versions.

Without the update to cedta() we get the telltale cedta() issues from here:

data.table/inst/tests/other.Rraw

Lines 227 to 229 in fae95de

	# example(":=", local=TRUE) triggered cedta==FALSE and then error, #2972
	test(14.1, {example(':=', package='data.table', local=TRUE, echo=FALSE); TRUE})
	test(14.2, {example('CJ', package='data.table', local=TRUE, echo=FALSE); TRUE})

Expected: 
Observed: [ was called on a data.table in an environment that is not data.table-aware (i.e. cedta()), but ':=' was used, implying the owner of this call really intended for data.table methods to be called. See vignette('datatable-importing') for details on properly importing data.table.
Test 14.2 produced 1 errors but expected 0
Expected: 
Observed: could not find function "J"

AIUI this is because of #7162 adding one more item on the call stack during execution of example().

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.01%. Comparing base (dcaa87c) to head (5cd4000).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7855   +/-   ##
=======================================
  Coverage   99.01%   99.01%           
=======================================
  Files          88       88           
  Lines       17286    17286           
=======================================
  Hits        17116    17116           
  Misses        170      170           

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[github-actions]

• HEAD=example-local-redux much slower for Date-IDate regression in #7213
• HEAD=example-local-redux much faster for transform improved in #5493
• HEAD=example-local-redux slower P<0.001 for DT[by] max regression fixed in #7480
  

Generated via commit 5cd4000

Download link for the artifact containing the test results: ↓ atime-results.zip

Task	Duration
R setup and installing dependencies	2 minutes and 55 seconds
Installing different package versions	1 minutes and 14 seconds
Running and plotting the test cases	6 minutes and 20 seconds