Make a new test folder for variable tests. (#1302)

Variables (and soon, functions) are sort of weird because they can
occur at the top level and as statements. It's not clear where they
should go. Before this PR, they were in both top_level/ and statement/.
This moves them to their own test directory.

I'll be doing the same thing for functions too.

Author: munificent

test/README.md

@@ -63,12 +63,15 @@ These tests are all run by `short_format_test.dart`.
 The newer tall style tests are:
 
 ```
-expression/   - Test formatting expressions.
-invocation/   - Test formatting function and member invocations.
-member/       - Test formatting class/enum/extension/mixin member declarations.
-statement/    - Test formatting statements.
-top_level/    - Test formatting top-level declarations and directives.
-type/         - Test formatting type annotations.
+declaration/  - Typedef, class, enum, and extension declarations.
+expression/   - Expressions.
+invocation/   - Function and member invocations.
+member/       - Constructor, method, field, getter, and setter declarations.
+selection/    - Test preserving selection information.
+statement/    - Statements.
+top_level/    - Top-level directives.
+type/         - Type annotations.
+variable/     - Top-level and local variable declarations.
 ```
 
 These tests are all run by `tall_format_test.dart`.

test/tall_format_test.dart

@@ -15,6 +15,7 @@ void main() async {
   await testDirectory('statement', tall: true);
   await testDirectory('top_level', tall: true);
   await testDirectory('type', tall: true);
+  await testDirectory('variable', tall: true);
 
   // TODO(tall): The old formatter_test.dart has tests here for things like
   // trailing newlines. Port those over to the new style once it supports all