Makefile: Clearer warning on missing validation executable(s)

Instead of reporting

  total ................................................. 0/0

and passing (which node-tap seems to do when given missing files as
arguments), report:

  missing test executable; run 'make runtimetest validation-executables'

and error out.  Having a separate step to build the executables is
useful for:

* Not building them as root, which reduces the change of security
  issues by using as little privilege as possible.
* Not rebuilding them on each test, since we haven't told Make about
  the full dependency tree for the test executables.

The separate build step was introduced in e11b77f (validation: Use
prove(1) as a TAP harness, 2017-11-25, opencontainers#439), but I didn't do a good
job of motivating it in that commit message.

Signed-off-by: W. Trevor King <wking@tremily.us>

Makefile

@@ -40,6 +40,10 @@ clean:
 	rm -f oci-runtime-tool runtimetest *.1 $(VALIDATION_TESTS)
 
 localvalidation:
+	for EXECUTABLE in runtimetest $(VALIDATION_TESTS); \
+	do \
+		test -x "$${EXECUTABLE}"; \
+	done || (echo "missing test executable; run 'make runtimetest validation-executables'" >&2 && exit 1)
 	RUNTIME=$(RUNTIME) $(TAP) $(VALIDATION_TESTS)
 
 .PHONY: validation-executables