Allow multi-line search in helix logs

[dibarbet]

cc @jcouv

It would be useful to allow multi-line matches for the search regex in helix logs. There are cases where we want to narrow down things like a crash in a specific test, or a specific stack trace. For example I may want to search that the test run was aborted while X test is running. Dotnet test currently outputs these as separate lines

The active Test Run was aborted because the host process exited unexpectedly. Please inspect the call stack above, if available, to get more information about where the exception originated from.
The test running when the crash occurred: 
Microsoft.CodeAnalysis.CSharp.UnitTests.CodeGen.CodeGenTupleTests.TupleWithElementNamedWithDefaultName

Helix log search calls into https://github.com/jaredpar/runfo/blob/main/DevOps.Util.DotNet/DotNetQueryUtil.cs#L337 which does a line by line regex match

[jcouv]

We may also be able to get away with a more limited feature: search for lines following the "The test running when the crash occurred: " line.