Make tests more resilient to timing. (#7753)

These tests would fail sometimes when run locally. Since they happen
to also be the ones that are flaky on bots, maybe there's some
relationship, even though the failure mode on bots is different. In
any case, this removes a variable.

dev/automated_tests/flutter_test/test_async_utils_guarded_expectation.txt

diff --git a/dev/automated_tests/flutter_test/test_async_utils_guarded_expectation.txt b/dev/automated_tests/flutter_test/test_async_utils_guarded_expectation.txt
index ee5901b..4601a54 100644
--- a/dev/automated_tests/flutter_test/test_async_utils_guarded_expectation.txt
+++ b/dev/automated_tests/flutter_test/test_async_utils_guarded_expectation.txt
@@ -1,4 +1,4 @@
-.*(this line contains the test framework's output with the clock and so forth)?
+[^═]*(this line contains the test framework's output with the clock and so forth)?
 ══╡ EXCEPTION CAUGHT BY FLUTTER TEST FRAMEWORK ╞════════════════════════════════════════════════════
 The following assertion was thrown running a test:
 Guarded function conflict\. You must use "await" with all Future-returning test APIs\.

dev/automated_tests/flutter_test/test_async_utils_unguarded_expectation.txt

diff --git a/dev/automated_tests/flutter_test/test_async_utils_unguarded_expectation.txt b/dev/automated_tests/flutter_test/test_async_utils_unguarded_expectation.txt
index b07f651..de23568 100644
--- a/dev/automated_tests/flutter_test/test_async_utils_unguarded_expectation.txt
+++ b/dev/automated_tests/flutter_test/test_async_utils_unguarded_expectation.txt
@@ -1,4 +1,4 @@
-.*(this line contains the test framework's output with the clock and so forth)?
+[^═]*(this line contains the test framework's output with the clock and so forth)?
 ══╡ EXCEPTION CAUGHT BY FLUTTER TEST FRAMEWORK ╞════════════════════════════════════════════════════
 The following assertion was thrown running a test:
 Guarded function conflict\. You must use "await" with all Future-returning test APIs\.