Reorganize tests and benchmark scripts.

Mainly to get rid of one top level directory. But this will
also be useful when there are tests of the embedding API.

test/core/number/sqrt.wren

@@ -0,0 +1,10 @@
+IO.print(4.sqrt)        // expect: 2
+IO.print(1000000.sqrt)  // expect: 1000
+IO.print(1.sqrt)        // expect: 1
+IO.print((-0).sqrt)     // expect: -0
+IO.print(0.sqrt)        // expect: 0
+IO.print(2.sqrt)        // expect: 1.4142135623731
+
+IO.print((-4).sqrt.isNan) // expect: true
+
+// TODO: Tests for sin and cos.