[tests] Add analysis options file to migrated test directories

The intent of this change is to make editing the tests in these
directories more user friendly. When you open these directories in an
analyzer enabled editor it will produce results consistent with the
language features being used in the test.

Change-Id: I68e9cdc0e9fccfc360a2649261a5fdca042ec108
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/139560
Reviewed-by: Paul Berry <paulberry@google.com>
Reviewed-by: Bob Nystrom <rnystrom@google.com>
Commit-Queue: Nicholas Shahan <nshahan@google.com>

tests/corelib/analysis_options.yaml

@@ -0,0 +1,3 @@
+analyzer:
+  enable-experiment:
+  - non-nullable

tests/language/analysis_options.yaml

@@ -0,0 +1,3 @@
+analyzer:
+  enable-experiment:
+  - non-nullable

tests/lib/analysis_options.yaml

@@ -0,0 +1,3 @@
+analyzer:
+  enable-experiment:
+  - non-nullable