Allow debug mode by default. #659

Job	Run time
cs-stan / Coding Standard & Static Analysis	32s
testsuite / testsuite-linux (8.2, pgsql, highest, false)	44s
testsuite / testsuite-linux (8.2, mysql, highest, false)	55s
testsuite / testsuite-linux (8.3, sqlite, highest, false)	27s
testsuite / testsuite-linux (8.2, sqlite, lowest, false)	30s
testsuite / testsuite-linux (8.5, sqlite, highest, false)	39s
testsuite / testsuite-linux (8.4, mysql, highest, false)	51s
	4m 38s

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