os_fchown: enable 'change group' unittest on Travis-CI

Add section `before_install` in `.travis.yml` to create test group and
add current user to this group.
It is needed because by default user on Travis-CI belongs only to one
primary group derived from that user. So we have no alternative to
change group of the file.

.travis.yml

@@ -13,5 +13,13 @@ env:
     - CI_TARGET=clint
     - CI_TARGET=api-python
     - CI_TARGET=coverity
+before_install:
+  # Adds user to a dummy group.
+  # That allows to test changing the group of the file by `os_fchown`.
+  - sudo groupadd chown_test
+  - sudo usermod -a -G chown_test ${USER}
 script:
-  - sh -e "${CI_SCRIPTS}/${CI_TARGET}.sh"
+  # This will pass the environment variables down to a bash process which runs
+  # as $USER, while retaining the environment variables defined and belonging
+  # to secondary groups given above in usermod.
+  - sudo -E su ${USER} -c "sh -e \"${CI_SCRIPTS}/${CI_TARGET}.sh\""

test/unit/os/fs_spec.moon

@@ -188,7 +188,7 @@ describe 'fs function', ->
             new_gid = id_fd\read '*n'
           id_fd\close!
 
-          -- User can be a member of only one group (e.g., it is on Travis-CI).
+          -- User can be a member of only one group.
           -- In that case we can not perform this test.
           if new_gid
             eq 0, (os_fchown filename, -1, new_gid)