Add some unit tests for in-string eval

unittests/instring_eval.chai

@@ -0,0 +1,3 @@
+var bob = 5.5
+print("${bob}")
+print("val: ${5.5 + 2.5} and ${bob + 2.5}")

unittests/instring_eval.txt

@@ -0,0 +1,2 @@
+5.5
+val: 8 and 8