tests/tcg/sh4: disable another unreliable test

Given the other failures it looks like general thread handling on sh4
is sketchy. It fails more often on CI than on my developer machine
though. See https://gitlab.com/qemu-project/qemu/-/issues/856

 for more
details.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Cc: Yoshinori Sato <ysato@users.sourceforge.jp>
Cc: Laurent Vivier <laurent@vivier.eu>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20220204204335.1689602-17-alex.bennee@linaro.org>

tests/tcg/sh4/Makefile.target

@@ -18,3 +18,7 @@ run-linux-test: linux-test
 	$(call skip-test, $<, "BROKEN")
 run-plugin-linux-test-with-%:
 	$(call skip-test, $<, "BROKEN")
+
+# This test is currently unreliable: https://gitlab.com/qemu-project/qemu/-/issues/856
+run-plugin-threadcount-with-%:
+	$(call skip-test, $<, "BROKEN")