misc: Eliminate use of assert!((...).is_ok())

Asserting on .is_ok()/.is_err() leads to hard to debug failures (as if
the test fails, it will only say "assertion failed: false". We replace
these with `.unwrap()`, which also prints the exact error variant that
was unexpectedly encountered (we can to this these days thanks to
efforts to implement Display and Debug for our error types). If the
assert!((...).is_ok()) was followed by an .unwrap() anyway, we just drop
the assert.

Inspired by and quoted from @roypat.

Signed-off-by: Ruoqing He <heruoqing@iscas.ac.cn>

tests/integration.rs

@@ -362,13 +362,13 @@ fn _test_api_pause_resume(target_api: TargetApi, guest: Guest) {
         thread::sleep(std::time::Duration::new(2, 0));
 
         // SSH into the VM should fail
-        assert!(ssh_command_ip(
+        ssh_command_ip(
             "grep -c processor /proc/cpuinfo",
             &guest.network.guest_ip,
             2,
-            5
+            5,
         )
-        .is_err());
+        .unwrap_err();
 
         // Resume the VM
         assert!(target_api.remote_command("resume", None));
@@ -6580,7 +6580,7 @@ mod common_parallel {
             thread::sleep(std::time::Duration::new(5, 0));
 
             // Check the connection fails this time
-            assert!(guest2.ssh_command("nc -vz 172.100.0.1 12345").is_err());
+            guest2.ssh_command("nc -vz 172.100.0.1 12345").unwrap_err();
 
             // Add the OVS port back
             assert!(exec_host_command_status("ovs-vsctl add-port ovsbr0 vhost-user1 -- set Interface vhost-user1 type=dpdkvhostuserclient options:vhost-server-path=/tmp/dpdkvhostclient1").success());