Stop test-in-container trying to run other-OS binaries.

I noticed that, now that build-many-glibcs.py no longer copies glibc
sources, I was getting core dumps in my glibc source directories.  The
cause appears to be, from the i686-gnu build:

for dso in ` env LD_TRACE_LOADED_OBJECTS=1  \
        /scratch/jmyers/glibc-bot/build/glibcs/i686-gnu/glibc/elf/ld.so.1 \
        /scratch/jmyers/glibc-bot/build/glibcs/i686-gnu/glibc/testroot.pristine/bin/sh \
[...]
Segmentation fault (core dumped)

In this case, the x86 architecture means the binary executes, but
dumps core rather than actually working.

Anything involving running the newly built glibc should only be done
ifeq ($(run-built-tests),yes).  This patch conditions the relevant
part of the testroot setup accordingly.

Tested for x86_64, and with build-many-glibcs.py for i686-gnu.

	* Makefile ($(objpfx)testroot.pristine/install.stamp): Do not run
	dynamic linker unless [$(run-built-tests) = yes].