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Re-enable coverage upload and close remaining coverage gaps (#161)

* test: re-enable coverage upload and close remaining coverage gaps

The Coveralls upload step was gated on refs/heads/main, but the default
branch is master, so coverage has not been published since January 2024.
Point it at master.

Since Go 1.22 the untested example binary counted as 0% and pulled the
module total from ~95% to ~86%. main() is split into newHandler /
printInstructions (plus a new -addr flag) so a smoke test can run the
example circuits against an httptest server and check both /debug/vars
and /hystrix.stream. The background goroutines still panic if a circuit
misbehaves, so this doubles as an integration check.

New unit tests for previously uncovered paths:
- GeneralConfig CustomConfig merging (copy, no aliasing, receiver wins)
- Circuit/Manager Var output with rolling run + fallback stats, including
  a throttled fallback
- Configurable open/close logic receiving SetConfig{,Not}ThreadSafe
- ClosedToOpen.Prevent short-circuiting without opening
- ForcedClosed never transitioning to open, then opening once cleared
- nil Circuit/Manager receivers; SimpleBadRequest Cause/Unwrap
- rolling FallbackStats.Var and negative config sanitizing
- responsetimeslo Factory with per-circuit config and collectors
- hystrix Opener err_% in MarshalJSON
- metriceventstream hystrix property reporting and non-flushable writer
- faststats RollingSum and Atomic UnmarshalJSON error paths
- evar fallback for a Var whose String() is not JSON

Total statement coverage goes from 86.4% to ~97%; the root package,
closers/hystrix, metrics/rolling and metrics/responsetimeslo are at 100%.
No library (non-example) source changes.

* test: avoid repeated string literals flagged by goconst

Newer golangci-lint counts literals in _test.go files toward goconst's
per-package total, so the new tests tripped it for the expvar key names
already used twice in rolling.go and for a repeated value in
config_tes... (continued)

17 of 31 new or added lines in 1 file covered. (54.84%)

2245 of 2293 relevant lines covered (97.91%)

172003.42 hits per line