cmd/shfmt: test that -i=0 does not skip ignore=true from EditorConfig

The man page says:

    If any parser or printer flags are given to the tool, no
    EditorConfig formatting options will be used. A default like -i=0
    can be used for this purpose.

Since ignore=true is not a formatting option, and it's still useful
when formatting options are given as flags, it should still work then.
The docs said so, but we had no specific test for it.

For #1095.

cmd/shfmt/testdata/script/editorconfig.txtar

@@ -57,6 +57,13 @@ stdout 'regular\.sh'
 ! stdout 'ignored\.sh'
 ! stderr .
 
+# EditorConfig ignore=true properties are obeyed even when any formatting flags
+# are used, which cause formatting options from EditorConfig files to be skipped.
+exec shfmt -i=0 -l ignored
+stdout 'regular\.sh'
+! stdout 'ignored\.sh'
+! stderr .
+
 # Formatting files directly does not obey ignore=true properties by default.
 # Test the various modes in which shfmt can run.
 ! exec shfmt -l input.sh ignored/1_lone_ignored.sh ignored/third_party/bad_syntax_ignored.sh