Make INI syntax register as handler of .inf files

We need to type `inf` and `INF` as strings in `INI.sublime-syntax`,
otherwise `yaml-rust` interprets them as real numbers ("infinity") and
they do not get registered as file extensions:

    /Users/martin/src/yaml-rust # https://github.com/chyh1990/yaml-rust
    % cargo run --example dump_yaml ~/src/bat/assets/syntaxes/02_Extra/INI.sublime-syntax
    ---
    String("name"):
        String("INI")
    String("file_extensions"):
            String("ini")
            String("INI")
            Real("inf")
            Real("INF")
    ...

Also add a regression test.

tests/syntax-tests/highlighted/INI/test.inf

@@ -0,0 +1,5 @@
+[section]
+key=value
+# This file is just for testing that the INI syntax is registered to handle
+# the .inf file extension, it is not testing the syntax highlighting capabilities
+# of the INI syntax itself

tests/syntax-tests/source/INI/test.inf

@@ -0,0 +1,5 @@
+[section]
+key=value
+# This file is just for testing that the INI syntax is registered to handle
+# the .inf file extension, it is not testing the syntax highlighting capabilities
+# of the INI syntax itself