[ci] Increase test timeout for Mac_x64 build_tests shards (flutter#188804)

Increase `test_timeout_secs` to `2700` (45 minutes) for all five
`Mac_x64 build_tests` shards (`1_5` to `5_5`) in `.ci.yaml`.

The default test timeout is 30 minutes. Shard `2_5` has been taking
close to 29 minutes, leading to timeouts. Since test sharding
distribution shuffles when tests are added/removed, all five shards have
been updated.

Setting `test_timeout_secs` to `2700` provides a 15-minute buffer before
the global LUCI task timeout of 60 minutes, ensuring the test runner
itself times out and prints diagnostics.

Fixes flutter#188242

Author: bkonyi

.ci.yaml

@@ -3679,6 +3679,7 @@ targets:
     timeout: 60
     properties:
       add_recipes_cq: "true"
+      test_timeout_secs: "2700"
       dependencies: >-
         [
           {"dependency": "android_sdk", "version": "version:36v8unmodified"},
@@ -3697,6 +3698,7 @@ targets:
     timeout: 60
     properties:
       add_recipes_cq: "true"
+      test_timeout_secs: "2700"
       dependencies: >-
         [
           {"dependency": "android_sdk", "version": "version:36v8unmodified"},
@@ -3715,6 +3717,7 @@ targets:
     timeout: 60
     properties:
       add_recipes_cq: "true"
+      test_timeout_secs: "2700"
       dependencies: >-
         [
           {"dependency": "android_sdk", "version": "version:36v8unmodified"},
@@ -3733,6 +3736,7 @@ targets:
     timeout: 60
     properties:
       add_recipes_cq: "true"
+      test_timeout_secs: "2700"
       dependencies: >-
         [
           {"dependency": "android_sdk", "version": "version:36v8unmodified"},
@@ -3751,6 +3755,7 @@ targets:
     timeout: 60
     properties:
       add_recipes_cq: "true"
+      test_timeout_secs: "2700"
       dependencies: >-
         [
           {"dependency": "android_sdk", "version": "version:36v8unmodified"},