tests: add basic_regular_guest macro and with_kernel

Introduce a basic_regular_guest! macro in integration.rs
to reduce boilerplate when creating regular (non-CVM)
guest instances. Also add a with_kernel() builder method
to Guest in test_infra, allowing fluent configuration of
the kernel path.

Signed-off-by: Muminul Islam <muislam@microsoft.com>

test_infra/src/lib.rs

@@ -1018,6 +1018,11 @@ impl Guest {
         self
     }
 
+    pub fn with_kernel(mut self, kernel: String) -> Self {
+        self.kernel_path = Some(kernel);
+        self
+    }
+
     pub fn default_net_string(&self) -> String {
         format!(
             "tap=,mac={},ip={},mask=255.255.255.128",