tests: add "include-fixed" GCC directory to include directories

[diabonas]

On Arch Linux, the GCC version of limits.h is in the include-fixed instead of the include directory. It needs to be included in the include directories list, otherwise attempting to compile the test suite fails with the following error:

In file included from /usr/include/efivar/efivar-dp.h:22,
                 from /usr/include/efivar/efivar.h:238,
                 from include/test.h:51,
                 from shim.h:68,
                 from csv.c:6:
/usr/include/limits.h:124:16: fatal error: limits.h: No such file or directory
  124 | # include_next <limits.h>
      |                ^~~~~~~~~~
compilation terminated.

[frozencemetery]

A comment in test.mk indicating that this is for Arch would be nice. Otherwise, while have to add this isn't ideal, the change looks fine to me.

[diabonas]

A comment in test.mk indicating that this is for Arch would be nice.

Sure, done :) I don't think adding "inline" comments in a variable definition continued over multiple lines using backslashes is possible in a Makefile, so used a separate addtion to CFLAGS at the very end instead (which builds fine as well).

[frozencemetery]

Yeah, I think like this is fine. (If order became tricky, it could be another variable that gets added where you had it originally.)