chore: merge Tests for check flat case (#689)

* update function documentation and name to match js convention

* add additional documentation explaining what the function does

* add tests for checkFlatCase function

* fix standard.js errors

String/test/CheckFlatCase.test.js

@@ -0,0 +1,18 @@
+import { checkFlatCase } from '../CheckFlatCase'
+
+describe('checkFlatCase function', () => {
+  it('should return false when the input string is not in flatcase', () => {
+    const actual = checkFlatCase('this is not in flatcase')
+    expect(actual).toBe(false)
+  })
+
+  it('should return true when the input string is a single letter character', () => {
+    const actual = checkFlatCase('a')
+    expect(actual).toBe(true)
+  })
+
+  it('should return true when the input string is a string of lowercase letter characters with no spaces', () => {
+    const actual = checkFlatCase('abcdefghijklmnopqrstuvwxyz')
+    expect(actual).toBe(true)
+  })
+})