test(context): Added regression test for requiredFlagsError

This adds a test verifying that the requiredFlagsError does contain the long
option of the missing flag, instead of the short option and a space, which was
the old behaviour.

Signed-off-by: Ole Petter <ole.orhagen@northern.tech>
(cherry picked from commit f842187ebb32ee7d5109783d02fe9902b68ee54e)

context_test.go

@@ -534,6 +534,14 @@ func TestCheckRequiredFlags(t *testing.T) {
 			},
 			parseInput: []string{"-n", "asd", "-n", "qwe"},
 		},
+		{
+			testCase: "required_flag_with_short_alias_not_printed_on_error",
+			expectedAnError: true,
+			expectedErrorContents: []string{"Required flag \"names\" not set"},
+			flags: []Flag{
+				StringSliceFlag{Name: "names, n", Required: true},
+			},
+		},
 	}
 
 	for _, test := range tdata {