Launch early reachability tests once a minute, not once a second.

[nmathewson]

This fixes bug 40083, which was introduced in 9f31908 in
0.2.8.1-alpha.

[coveralls]

Pull Request Test Coverage Report for Build 9770

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 13 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.02%) to 59.227%

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Files with Coverage Reduction	New Missed Lines	%
src/feature/dirauth/shared_random.c	4	85.1%
src/feature/dirauth/dirvote.c	9	64.65%

Totals
Change from base Build 9738:	-0.02%
Covered Lines:	43320
Relevant Lines:	73142

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💛 - Coveralls