tests/qtest/libqtest.c: Check for g_setenv() failure

tests/qtest/libqtest.c: Check for g_setenv() failure

Coverity points out that g_setenv() can fail and we don't
check for this in qtest_inproc_init(). In practice this will
only fail if a memory allocation failed in setenv() or if
the caller passed an invalid architecture name (e.g. one
with an '=' in it), so rather than requiring the callsite
to check for failure, make g_setenv() failure fatal here,
similarly to what we did in commit aca68d95c515.

Resolves: Coverity CID 1497485
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-id: 20240312183810.557768-8-peter.maydell@linaro.org