o/devicestate: preseeding test cleanup

[stolowski]

• pass "classic" flag to deviceMgrBaseSuite to allow creating device manager with mocked classic/core upfront. This doesn't prevent tests from overriding MockOnClassic later in the test but is important in case DeviceManager needs to know if it's on classic early in its init (important for core20/preseeding tests).
• drop TestPreseedHappy which was pointless/misleading as it was actually run against core; TestPreseedOnClassicHappy covers real case.
• renamed test suites from preseed* to preseeding*
• some minor cleanups and sanity checks for classic.

[mardy]

LGTM, just a question.

[pedronis]

thank you, two small remarks

[codecov-commenter]

Codecov Report

Merging #11398 (dc86b35) into master (b1270c5) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #11398   +/-   ##
=======================================
  Coverage   78.38%   78.38%           
=======================================
  Files         931      931           
  Lines      106738   106738           
=======================================
+ Hits        83664    83667    +3     
+ Misses      17870    17868    -2     
+ Partials     5204     5203    -1     

Flag	Coverage Δ
unittests	78.38% <ø> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
overlord/devicestate/firstboot.go	87.58% <0.00%> (-1.04%)	⬇️
overlord/hookstate/hookmgr.go	74.67% <0.00%> (-0.65%)	⬇️
overlord/ifacestate/handlers.go	65.02% <0.00%> (+0.14%)	⬆️
daemon/api_connections.go	93.58% <0.00%> (+0.53%)	⬆️
store/cache.go	71.15% <0.00%> (+1.92%)	⬆️
osutil/synctree.go	79.24% <0.00%> (+2.83%)	⬆️

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