tests: fix fluctuations in test coverage (#647)

This basically started once our integration tests caused the overall
test time to raise above 10 minutes, causing some coverage data to be
dropped because SimpleCov believed it to be stale.

Library/Homebrew/test/.simplecov

@@ -7,6 +7,11 @@ SimpleCov.start do
   coverage_dir File.expand_path("#{tests_path}/coverage")
   root File.expand_path("#{tests_path}/..")
 
+  # We manage the result cache ourselves and the default of 10 minutes can be
+  # too low (particularly on Travis CI), causing results from some integration
+  # tests to be dropped. This causes random fluctuations in test coverage.
+  merge_timeout 86400
+
   add_filter "/Homebrew/compat/"
   add_filter "/Homebrew/test/"
   add_filter "/Homebrew/vendor/"