Server USB tests fail 100% of the time

Unfortuately, we missed this because our test machines in the labs don't have USB sticks plugged in, and when doing this testing for cert, we use Stable.

this commit from October lived in Dev and unfortunately we didn't catch it until now:
commit 41ef34811c08df2d0b78082dc341fa622e876ce7
Author: Lukas Waymann <email address hidden>
Date: Fri Oct 30 13:33:37 2020 +0800

    Skip after-suspend-usb*/storage-preinserted when DUT lacks USB drive

this adds manifest lines to the usb preinserted tests:
bladernr@galactica:~/development/plainbox-provider-checkbox/units/usb$ git diff 6bcf32590a44279c2abc02fa0fcedb746c00b1ea 41ef34811c08df2d0b78082dc341fa622e876ce7
diff --git a/units/usb/usb.pxu b/units/usb/usb.pxu
index 0b923cb..b73d36c 100644
--- a/units/usb/usb.pxu
+++ b/units/usb/usb.pxu
@@ -244,6 +244,7 @@ requires:
  cpuinfo.platform != 's390x'
  package.name == 'udisks2' or snap.name == 'udisks2'
  package.name == 'udisks2' or (snap.name == 'core' and int(snap.revision) >= 1804)
+ manifest.has_usb_storage == 'True'
 _summary:
  Test USB 2.0 or 1.1 ports
 _description:
@@ -261,6 +262,7 @@ requires:
  usb.usb3 == 'supported'
  package.name == 'udisks2' or snap.name == 'udisks2'
  package.name == 'udisks2' or (snap.name == 'core' and int(snap.revision) >= 1804)
+ manifest.has_usb_storage == 'True'
 estimated_duration: 45.0
 command: removable_storage_test.py -l usb && timeout 300 removable_storage_test.py -s 268400000 -m 500000000 usb --driver xhci_hcd
 _summary:

and this breaks server cert across the board.