[testing] Format logs with more distinctive separators
[testing] Format logs with more distinctive separators

The output of a compare-results run showing logs will enclose
the failing test name in a box, as in:

/=======================================================================================\
| dart2js_extra/no_such_method_test is new and failed (CompileTimeError, expected Pass) |
\=======================================================================================/

--- Command "dart2js" (took 244ms):
DART_CONFIGURATION=ReleaseX64 out/ReleaseX64/dart-sdk/bin/dart2js --generate-code-with-compile-time-errors --test-mode --packages=/b/s/w/ir/cache/builder/sdk/.packages --out=/b/s/w/ir/cache/builder/sdk/out/ReleaseX64/generated_compilations/dart2js-sdk/tests_compiler_dart2js_extra_no_such_method_test/test.js /b/s/w/ir/cache/builder/sdk/tests/compiler/dart2js_extra/no_such_method_test.dart --preview-dart-2

exit code:
1

Change-Id: Ic63fa41381bab320636340c1e7919e22cd424c1d
Reviewed-on: https://dart-review.googlesource.com/c/86925
Reviewed-by: Johnni Winther <0f64ef784d57ffb064b634040f9eeba0d33d0be4@google.com>
Reviewed-by: Jonas Termansen <092ec58085ae5e368bc39d18752740ff81a2d09c@google.com>

dartino/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk