test suite: In ad-hoc runs, honour DGIT_TEST_RETRY_COUNT

This lets us retry each failed test.

This is beneficial because the gnupg race is sort-of independent.
When we want to test every commit, the success probability before
is
  ( 1 - P(gnupg causes test fail))        ) ^ (number of tests)
for each commit.  With this change it's
  ( 1 - P(gnupg causes test fail)^retries ) ^ (number of tests)
which is much much better.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>

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