fix test sharding (#156768)

Also cleans up https://github.com/flutter/flutter/issues/156762

<details>

<summary> Pre-launch checklist </summary> 

</details>

.ci.yaml

@@ -5361,7 +5361,6 @@ targets:
       - .ci.yaml
 
   - name: Windows build_tests_1_8
-    bringup: true
     recipe: flutter/flutter_drone
     timeout: 60
     properties:
@@ -5380,7 +5379,6 @@ targets:
         ["framework", "hostonly", "shard", "windows"]
 
   - name: Windows build_tests_2_8
-    bringup: true
     recipe: flutter/flutter_drone
     timeout: 60
     properties:
@@ -5399,7 +5397,6 @@ targets:
         ["framework", "hostonly", "shard", "windows"]
 
   - name: Windows build_tests_3_8
-    bringup: true
     recipe: flutter/flutter_drone
     timeout: 60
     properties:
@@ -5418,7 +5415,6 @@ targets:
         ["framework", "hostonly", "shard", "windows"]
 
   - name: Windows build_tests_4_8
-    bringup: true
     recipe: flutter/flutter_drone
     timeout: 60
     properties:
@@ -5437,7 +5433,6 @@ targets:
         ["framework", "hostonly", "shard", "windows"]
 
   - name: Windows build_tests_5_8
-    bringup: true
     recipe: flutter/flutter_drone
     timeout: 60
     properties:
@@ -5456,7 +5451,6 @@ targets:
         ["framework", "hostonly", "shard", "windows"]
 
   - name: Windows build_tests_6_8
-    bringup: true
     recipe: flutter/flutter_drone
     timeout: 60
     properties:
@@ -5475,7 +5469,6 @@ targets:
         ["framework", "hostonly", "shard", "windows"]
 
   - name: Windows build_tests_7_8
-    bringup: true
     recipe: flutter/flutter_drone
     timeout: 60
     properties:
@@ -5494,7 +5487,6 @@ targets:
         ["framework", "hostonly", "shard", "windows"]
 
   - name: Windows build_tests_8_8
-    bringup: true
     recipe: flutter/flutter_drone
     timeout: 60
     properties: