Make the docker check more specific

Docker on a mac might be available, but may not be runnng. The
way the original test was written is linux-specific anyway, so
just hoist the check into the DOCKER declaration

test/sharness/lib/test-lib.sh

@@ -52,7 +52,7 @@ TEST_OS="$(uname -s | tr '[a-z]' '[A-Z]')"
 # grab + output options
 test "$TEST_NO_FUSE" != 1 && test_set_prereq FUSE
 test "$TEST_EXPENSIVE" = 1 && test_set_prereq EXPENSIVE
-test "$TEST_NO_DOCKER" != 1 && type docker >/dev/null 2>&1 && test_set_prereq DOCKER
+test "$TEST_NO_DOCKER" != 1 && type docker >/dev/null 2>&1 && groups | egrep "\bdocker\b" && test_set_prereq DOCKER
 test "$TEST_NO_PLUGIN" != 1 && test "$TEST_OS" = "LINUX" && test_set_prereq PLUGIN
 
 # this may not be available, skip a few dependent tests

test/sharness/t0300-docker-image.sh

@@ -23,10 +23,6 @@ test_expect_success "'docker --version' output looks good" '
   egrep "^Docker version" actual
 '
 
-test_expect_success "current user is in the 'docker' group" '
-  groups | egrep "\bdocker\b"
-'
-
 TEST_TRASH_DIR=$(pwd)
 TEST_SCRIPTS_DIR=$(dirname "$TEST_TRASH_DIR")
 TEST_TESTS_DIR=$(dirname "$TEST_SCRIPTS_DIR")

test/sharness/t0301-docker-migrate.sh

@@ -10,7 +10,7 @@ test_description="Test docker image migration"
 
 # if in travis CI on OSX, docker is not available
 if ! test_have_prereq DOCKER; then
-  skip_all='skipping docker tests, docker not available'
+  skip_all='skipping '$test_description', docker not available'
 
   test_done
 fi