mk/tests.mk: find -perm +mode is deprecated

[Munksgaard]

In mk/tests.mk we use the command find -perm +a+x to find executables. The syntax +a+x has been deprecated since 2005 and have been removed from newer versions of find (mine is find (GNU findutils) 4.5.14), and should be replaced with /a+x.

[alexcrichton]

Er, didn't actually land in the rollup, just didn't take the PR out of it, sorry!