Sort the tests alphabetically before adding them to the Kyuafiles

This is being done to aid in debugging test runs, in the event the
output shifts due to refactored Makefiles, added tests, etc.

MFC after:	1 month

share/mk/bsd.test.mk

@@ -66,6 +66,10 @@ _TESTS=
 .include <plain.test.mk>
 .include <tap.test.mk>
 
+# Sort the tests alphabetically, so the results are deterministically formed
+# across runs.
+_TESTS:=	${_TESTS:O}
+
 # kyua automatically descends directories; only run make check on the
 # top-level directory
 .if !make(check)