[infra] Make the sanitizer a first-class status variable.

Makes the TSAN skips apply to
 `test.py -mrelease --sanitizer=tsan`
and not just
 `test.py -n vm-tsan-linux-release-x64`.

Also makes the timeouts agree.

Change-Id: I10315e754a4ebb3020f3c2f6cecfac6b77e77a9b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/311828
Reviewed-by: Alexander Thomas <athom@google.com>
Commit-Queue: Ryan Macnak <rmacnak@google.com>

samples/samples.status

@@ -2,7 +2,7 @@
 # for details. All rights reserved. Use of this source code is governed by a
 # BSD-style license that can be found in the LICENSE file.
 
-[ $builder_tag == asan ]
+[ $sanitizer == asan ]
 ffi/samples_test: SkipByDesign # FFI skips, see ffi.status
 
 [ $simulator ]