tests: command: Fix build on ppc64/aarch64

Commit ca10bb04 introduced a new test that fails to build
on at least some architectures:

  commandtest.c: In function 'test25':
  commandtest.c:1121:5: error: comparison is always true due to
                        limited range of data type [-Werror=type-limits]
    if (rv >= 0) {
    ^

Change the type of 'rv' from char to int, which is the proper
return type for virCommandExec() anyway.

tests/commandtest.c

@@ -1068,7 +1068,7 @@ static int test25(const void *unused ATTRIBUTE_UNUSED)
 {
     int ret = -1;
     int pipeFD[2] = { -1, -1};
-    char rv = 0;
+    int rv = 0;
     ssize_t tries = 100;
     pid_t pid;