m/t/installer: deflake by silencing kernel logs

The installer test was failing due to the success flag getting broken
in two by the kernel logs, like this:
>TestOS launched successfully! _TESTOS_LAUNCH_SUC[    0.963289] kvm:
>exiting hardware virtualizationCESS_

This change addresses this problem by setting an adequate loglevel for
both the installer and testos kernels.

m/t/installer/BUILD.bazel now defines its own unified kernel image
target with a custom kernel cmdline needed for test purposes.

Change-Id: I43547b6c81f8a1ebc4facb841a600b155af581df
Reviewed-on: https://review.monogon.dev/c/monogon/+/464
Reviewed-by: Sergiusz Bazanski <serge@monogon.tech>
diff --git a/metropolis/test/installer/main.go b/metropolis/test/installer/main.go
index a5bac64..34c4d31 100644
--- a/metropolis/test/installer/main.go
+++ b/metropolis/test/installer/main.go
@@ -156,7 +156,7 @@
 		// side effects, it must be just a filename.
 		src string
 	}{
-		{&installerEFIPayload, true, "metropolis/node/installer/kernel.efi"},
+		{&installerEFIPayload, true, "metropolis/test/installer/kernel.efi"},
 		{&testOSBundle, true, "metropolis/test/installer/testos/testos_bundle.zip"},
 		{&installerImage, false, "installer.img"},
 		{&nodeStorage, false, "stor.img"},