t/t0005-exec.t: Fix corner case in test for file not found

The fake file 'nosuchprocess' does not have a relative or absolute
path specified.  So the file will be searched for in all
directories in the user's PATH.  If the user does not have access
to one of the directories in their PATH, an EACCES error can occur
instead of ENOENT, leading to a failed test.  So put "./" in front
of 'nosuchprocess' to ensure it is not searched for in PATH.

t/t0005-exec.t

@@ -77,15 +77,15 @@ EOF
 '
 
 test_expect_success 'flux exec exits with code 127 for file not found' '
-	test_expect_code 127 run_timeout 2 flux exec nosuchprocess
+	test_expect_code 127 run_timeout 2 flux exec ./nosuchprocess
 '
 
 test_expect_success 'flux exec exits with code 126 for non executable' '
 	test_expect_code 126 flux exec /dev/null
 '
 
 test_expect_success 'flux exec exits with code 68 (EX_NOHOST) for rank not found' '
-	test_expect_code 68 run_timeout 2 flux exec -r 1000 nosuchprocess
+	test_expect_code 68 run_timeout 2 flux exec -r 1000 ./nosuchprocess
 '
 test_expect_success 'flux exec passes non-zero exit status' '
 	test_expect_code 2 flux exec sh -c "exit 2" &&