Fix test about curve

__mocks__/d3.js

@@ -32,3 +32,7 @@ export const select = function () {
 export const selectAll = function () {
   return new NewD3()
 }
+
+export const curveBasis = 'basis'
+export const curveLinear = 'linear'
+export const curveCardinal = 'cardinal'

src/mermaid.spec.js

@@ -118,7 +118,7 @@ describe('when using mermaid and ', function () {
           expect(start).toBe('A')
           expect(end).toBe('B')
           expect(options.arrowhead).toBe('none')
-          expect(options.lineInterpolate).toBe('basis')
+          expect(options.curve).toBe('basis') // mocked as string
         }
       }