maint: add a missed fadvise-tests module

* gl/modules/fadvise-tests: Add the module previously missed
in commit 63b5e816, 2010-07-14, "fadvise: new module ...".
* gl/tests/test-fadvise.c: Add a comment as to why we don't
check return values.

gl/modules/fadvise-tests

@@ -0,0 +1,10 @@
+Files:
+tests/test-fadvise.c
+
+Depends-on:
+
+configure.ac:
+
+Makefile.am:
+TESTS += test-fadvise
+check_PROGRAMS += test-fadvise

gl/tests/test-fadvise.c

@@ -21,6 +21,13 @@
 
 #include "fadvise.h"
 
+/* We ignore any errors as these hints are only advisory.
+ * There is the chance one can pass invalid ADVICE, which will
+ * not be indicated, but given the simplicity of the interface
+ * this is unlikely.  Also not returning errors allows the
+ * unconditional passing of descriptors to non standard files,
+ * which will just be ignored if unsupported.  */
+
 int
 main (void)
 {