Replace NewEndpointFromString with ParseEndpoint, improve tests

[Geod24]

NewEndpointFromString was primarily used from tests and had a different semantic
than the more complere ParseEndpoint.
Since having two functions to convert a string to an Endpoint is a bad sign,
the lesser function (with the longest name) was removed.
In addition, since many (mosts?) tests seemed to ignore the error return value,
a test-only MustParseEndpoint was introduced, as ignoring a return value
is a deadly sin.

[codecov-io]

Codecov Report

Merging #921 into master will decrease coverage by 0.05%. The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #921      +/-   ##
==========================================
- Coverage    60.8%   60.75%   -0.06%     
==========================================
  Files         159      159              
  Lines       11262    11260       -2     
==========================================
- Hits         6848     6841       -7     
- Misses       3609     3615       +6     
+ Partials      805      804       -1

Flag	Coverage Δ
#integration_tests_long_term	44.09% <0%> (-0.06%)	:arrow_down:
#integration_tests_node	40.56% <0%> (-0.06%)	:arrow_down:
#integration_tests_retry	37.72% <0%> (-0.11%)	:arrow_down:
#unittests	47.4% <0%> (ø)	:arrow_up:

Impacted Files	Coverage Δ
lib/common/net.go	37% <ø> (-3.3%)	:arrow_down:
cmd/sebak/cmd/run.go	56.92% <0%> (ø)	:arrow_up:
lib/common/test.go	25% <0%> (-4.63%)	:arrow_down:

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