Fix and enable testing stdlib Collection instances.

Top-level entry points fully testing a collection instance:
check${Traversal}Collection

One level of recursion into all slices of the collection instance
O(n^2). (Not combinatorial).

Previously, checkCollection() did nothing. So much of the testing infrastructure was inactive. Now it runs all forward collection tests.

Fixes a bug in subscriptRangeTests.

The UnsafeRawBufferPointer and Data collection testing is disabled and
will be fixed in the following commit.

validation-test/stdlib/Unicode.swift.gyb

@@ -1607,7 +1607,7 @@ func checkUTF8View(_ expected: [UInt8], _ subject: String,
 
 func checkUTF16View(_ expected: [UInt16], _ subject: String,
     _ stackTrace: SourceLocStack) {
-  checkSliceableWithBidirectionalIndex(expected, subject.utf16)
+  checkBidirectionalCollection(expected, subject.utf16)
 }
 
 func forStringsWithUnpairedSurrogates(_ checkClosure: (UTF16Test, String) -> Void) {
@@ -1848,7 +1848,7 @@ StringCookedViews.test("UnicodeScalars").forEach(in: utfTests) {
   test in
 
   let subject = NonContiguousNSString(test.utf32) as String
-  checkSliceableWithBidirectionalIndex(
+  checkBidirectionalCollection(
     test.unicodeScalars, subject.unicodeScalars)
 }
 
@@ -1858,7 +1858,7 @@ StringCookedViews.test("UnicodeScalars/StringsWithUnpairedSurrogates") {
     let expectedScalars = (test.scalarsHead + test.scalarsRepairedTail).map {
       UnicodeScalar($0)!
     }
-    checkSliceableWithBidirectionalIndex(
+    checkBidirectionalCollection(
       expectedScalars, subject.unicodeScalars)
   }
 }