Fix tests depending on configuration flag

[farazdagi]

What type of PR is this?

Bug fix

What does this PR do? Why is it needed?

• properly handles nil values (returns errors instead of segfault)
• number of tests are updated (most changes are just asserting proper config flag)

Which issues(s) does this PR fix?

Fixes #5648

Other notes for review

• There's no unhandled featureconfig.Init calls in tests anymore, I've fixed all tests - with one exception TestServer_ListBlocks_Pagination - it is marked as skipped, and I will create a separate issue for it
• I've moved config setting from init() functions to TestMain() (there were two such ocurrences)
• New method added to featureconfig:

// InitWithReset sets the global config and returns function that is used to reset configuration.
func InitWithReset(c *Flags) func() {
	resetFunc := func() {
		Init(&Flags{})
	}
	Init(c)
	return resetFunc
}

Please review how it is used, as it will be preferred way to set config params in tests (so suggestions are welcome):

resetCfg := featureconfig.InitWithReset(&featureconfig.Flags{NewStateMgmt:true})
defer resetCfg()

[codecov]

Codecov Report

Merging #5650 into master will decrease coverage by 1.86%.
The diff coverage is 38.46%.

@@            Coverage Diff             @@
##           master    #5650      +/-   ##
==========================================
- Coverage   22.94%   21.08%   -1.87%     
==========================================
  Files         268      179      -89     
  Lines       23003    16034    -6969     
==========================================
- Hits         5279     3381    -1898     
+ Misses      16856    12096    -4760     
+ Partials      868      557     -311