Add test suite for find.turn.point with gaussian mixture data

[Copilot]

Implements comprehensive test coverage for find.turn.point and its helper functions using simulated gaussian mixture distributions.

Test Coverage

• Single and multi-component gaussians: 1, 2, and 3-peak mixtures with varying separation, overlap, and amplitude
• Edge cases: constant values, NA filtering
• Helper functions: msExtrema and find_local_maxima validation

Example Test Pattern

test_that("find.turn.point identifies two-component gaussian mixture", {
  # Create mixture of two gaussians
  x <- seq(-10, 10, length.out = 400)
  y <- 0.6 * dnorm(x, mean = -3, sd = 1) + 0.4 * dnorm(x, mean = 3, sd = 1.5)
  
  result <- find.turn.point(y)
  
  # Validates 2 peaks detected, valleys at boundaries and between peaks
  expect_equal(length(result$pks), 2)
  expect_true(length(result$vlys) >= 3)
}

All test data generated via dnorm() for reproducible scenarios.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Implement test for the find.turn.point function using simulated gaussian mixture data</issue_title>
<issue_description></issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Implement test for the find.turn.point function using simulated gaussian mixture data #251

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.