test/eject: guard asan LD_PRELOAD with use-system-commands check

test/eject: guard asan LD_PRELOAD with use-system-commands check

This test tries to add asan to LD_PRELOAD because the just-built eject
will call the host /bin/umount, and apparently asan doesn't like this.

However, if ldd isn't present, this fails as the path to asan is the
error message saying that ldd isn't present.

As the asan workaround is only needed when executing the binaries that
have just been built and not the system binaries, only use it if the
test is on the built binaries.

Closes #1373.

Signed-off-by: Ross Burton <ross.burton@arm.com>