test-bot: fix coverage on Travis CI

Make sure to call `brew tests` only once with `--coverage` to avoid
expensive multiple runs and to prevent later runs from overwriting
previously sent results to Coveralls. (The previous setup overwrote the
results from a regular run with results from the `--generic` run.)

The `--no-compat` variant without any other options specified seemed to
be the most appropriate for a coverage report.

Closes Homebrew#546.

Signed-off-by: Martin Afanasjew <martin@afanasjew.de>

Library/Homebrew/dev-cmd/test-bot.rb

@@ -659,14 +659,15 @@ def homebrew
 
       if @tap.nil?
         tests_args = []
+        tests_args_coverage = []
         if RUBY_TWO
           tests_args << "--official-cmd-taps"
-          tests_args << "--coverage" if ENV["TRAVIS"]
+          tests_args_coverage << "--coverage" if ENV["TRAVIS"]
         end
         test "brew", "tests", *tests_args
         test "brew", "tests", "--generic", "--only=integration_cmds",
                               *tests_args
-        test "brew", "tests", "--no-compat"
+        test "brew", "tests", "--no-compat", *tests_args_coverage
         test "brew", "readall", "--syntax"
         # test update from origin/master to current commit.
         test "brew", "update-test"