Consolidate test helpers into internal/test package

[peterstace]

Description

Consolidates scattered test helper functions into the shared internal/test
package, reducing duplication across geom, geos, and internal/cartodemo
test files. The main geom/util_test.go file (324 lines) is removed entirely,
with its helpers either moved to internal/test or inlined where they were
single-use.

New helpers added to internal/test: Eq, GT, LT, True, False,
Panics, Err, ErrIs, ErrAs, ExactEquals, NotExactEquals,
ExactEqualsWKT, DeepEqual, NotDeepEqual, ApproxEqual, ImageToPNG,
ReadFile.

Also adds a NewEnvelopeXY constructor for building an Envelope from variadic
x/y coordinate pairs, following the existing New*XY pattern. This was needed
to replace a test-only envFromRaw helper with a proper API.

A tracking document (test-helper-consolidation.md) is included to capture
remaining consolidation opportunities.

Check List

Have you:

• Added unit tests?

  Yes — existing tests were updated to use the consolidated helpers, and the new
  NewEnvelopeXY constructor has test coverage in geom/ctor_from_coords_test.go.

• Add cmprefimpl tests? (if appropriate?)

  N/A — this is a test infrastructure change. The cmprefimpl package was updated
  to use the shared helpers where applicable.

• Updated release notes? (if appropriate?)

  Yes — CHANGELOG.md updated with the new NewEnvelopeXY constructor.

• Updated the README.md (if new functionality is added?)

  N/A — NewEnvelopeXY follows an existing pattern already documented.

Related Issue

• No related issue.

Benchmark Results

• N/A — this is a test-only refactor with no production code changes other than
  the small NewEnvelopeXY addition. No performance impact expected.