Added test for combining case insensitive and negated patterns (fixes #1678)

internal/fnmatch/fnmatch_test.go

@@ -55,6 +55,7 @@ var testcases = []testcase{
 	{"foo.txt", "foo.TXT", CaseFold, true},
 	{"(?i)foo.txt", "foo.TXT", 0, true},
 	{"(?i)**foo.txt", "/dev/tmp/foo.TXT", 0, true},
+	{"(?i)!**foo.txt", "/dev/tmp/foo.TXT", 0, false},
 
 	// These characters are literals in glob, but not in regexp.
 	{"hey$hello", "hey$hello", 0, true},