Add unit tests for math/function.

tests/math/function.bats

@@ -0,0 +1,15 @@
+#!/usr/bin/env bats
+
+source bashlet math/function
+
+@test "math/function: roughly estimate a square root" {
+  result="$(bash$$ function rough_sqrt 50)"
+  expected=8
+  [[ $result -eq $expected ]]
+}
+
+@test "math/function: calculate a square root" {
+  result="$(bash$$ function sqrt 64)"
+  expected=8
+  [[ $result -eq $expected ]]
+}