tests: Unify style of test skipping code

Prefer 'return EXIT_AM_SKIP' over 'exit(EXIT_AM_SKIP)'.

Prefer 'int main(void)' over 'int main(int argc, char **argv)'.

Fix mymain signature in commandtest and nodeinfotest.

tests/qemuxml2xmltest.c

@@ -206,6 +206,10 @@ VIRT_TEST_MAIN(mymain)
 
 #else
 
-int main (void) { exit (EXIT_AM_SKIP); }
+int
+main(void)
+{
+    return EXIT_AM_SKIP;
+}
 
 #endif /* WITH_QEMU */