tests/tcg: Add SIGRTMIN/SIGRTMAX test

tests/tcg: Add SIGRTMIN/SIGRTMAX test

Test the lowest and the highest real-time signals. This requires
configuring the real-time signal mapping, and therefore some knowledge
about the host. To this end, pass the emulator path in the QEMU
environment variable to all tests (this should not disturb the existing
ones), and assume that all hosts have signals 36-39 available.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Message-ID: <20241029232211.206766-3-iii@linux.ibm.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>