Fix graph printing in TDAG / CDAG generation tests

[fknorr]

Previously with --print-graphs, dot graphs were written to the log inside TDAG / CDAG tests, but test logs are captured and discarded on success since #234, breaking this command-line option.

This PR updates TDAG / CDAG tests to write dot graphs to stdout like already done for IDAG tests. Test names are included in the dot title, and no additional context information is printed alongside. The user / developer is expected to invoke test executables with render-graphs.py.

While at it, I've taken the opportunity to rename test_utils::print_graphs to g_print_graphs to highlight that it's an unprotected global.

[github-actions[bot]]

Check-perf-impact results: (7b849de16ff11660b98988ab0b032db7)

:question: No new benchmark data submitted. :question:
Please re-run the microbenchmarks and include the results if your commit could potentially affect performance.

[coveralls]

Pull Request Test Coverage Report for Build 10006864064

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.009%) to 93.075%

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Totals
Change from base Build 9992874606:	-0.009%
Covered Lines:	7231
Relevant Lines:	7507

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💛 - Coveralls