Add test suite for smart expression selector

Add test suite for smart expression selector (the small expression list
popup in e.g. Extract Variable.)

The test file format is similar to that in JetNameSuggesterTest: a .kt
file with the usual <caret> marker specifying the place where the smart
selector will be run. Then, the last comment in the file should contain
the expected outcome.

idea/testData/smartSelection/simple.kt

@@ -0,0 +1,9 @@
+fun main(args: Array<String>) {
+    print(Math.pow(1.<caret>0 + 1, Math.abs(-42.0)))
+}
+/*
+1.0
+1.0 + 1
+Math.pow(1.0 + 1, Math.abs(-42.0))
+print(Math.pow(1.0 + 1, Math.abs(-42.0)))
+*/