tests: Correctly skip qtest on non-POSIX hosts

qtest test cases only work on POSIX hosts.  The following line only
defines dependencies for qtest binaries on POSIX hosts:

  check-qtest-$(CONFIG_POSIX)=$(foreach TARGET,$(TARGETS),$(check-qtest-$(TARGET)-y))

But the QTEST_TARGETS definition earlier in the Makefile fails to check
CONFIG_POSIX.  This causes make targets to be generated for qtest test
cases even though we don't know how to build the binaries.

The following error message is printed when trying to run gtester on a
binary that was never built:

  GLib-WARNING **: Failed to execute test binary: tests/endianness-test.exe: Failed to execute child process "tests/endianness-test.exe" (No such file or directory)

This patch makes QTEST_TARGETS empty on non-POSIX hosts.  This prevents
the targets from being generated.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
