[browser] Increase timeout for System.Memory.Tests and System.Private.Xml.Tests

[maraf]

There doesn't seem to be any relevant change in tests directly. Trying to increase timeout

Related to #102955
Related to #102957
Related to #103060
Related to #103061
Related to #103246

[dotnet-policy-service]

Tagging subscribers to this area: @directhex, @matouskozak
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[maraf]

/azp run runtime

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[maraf]

/azp run runtime

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[maraf]

/azp run runtime

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[akoeplinger]

@dotnet-policy-service rerun

[maraf]

Firefox MT https://helixre107v0xdcypoyl9e7f.blob.core.windows.net/dotnet-runtime-refs-pull-103221-merge-e06ce049c13b41cfa8/WasmTestOnFirefox-MT-System.Buffers.Tests/1/console.dd1a5cfc.log?helixlogtype=result

[08:25:02] info: [FAIL] System.Buffers.ArrayPool.Tests.ArrayPoolUnitTests.RentingSpecificLengthsYieldsExpectedLengths(requestedMinimum: 2097152, expectedLength: 2097152)
[08:25:02] info: System.OutOfMemoryException : Out of memory
[08:25:02] info:    at System.Buffers.ConfigurableArrayPool`1[[System.String, System.Private.CoreLib, Version=9.0.0.0, Culture=neutral, PublicKeyToken=7cec85d7bea7798e]].Rent(Int32 minimumLength)
[08:25:02] info:    at System.Buffers.ArrayPool.Tests.ArrayPoolUnitTests.RentingSpecificLengthsYieldsExpectedLengths(Int32 requestedMinimum, Int32 expectedLength) in /_/src/libraries/System.Runtime/tests/System.Buffers.Tests/ArrayPool/UnitTests.cs:line 336
[08:25:02] info:    at System.Object.InvokeStub_ArrayPoolUnitTests.RentingSpecificLengthsYieldsExpectedLengths(Object , Span`1 )
[08:25:02] info:    at System.Reflection.MethodBaseInvoker.InvokeWithFewArgs(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)
[08:25:02] info: System.Buffers.ArrayPool.Tests.ArrayPoolUnitTests 37.0964753 ms
[08:25:02] info: Received expected 25517 of /root/helix/work/workitem/uploads/xharness-output/testResults.xml
[08:25:02] info: Finished writing 25517 bytes of RESULTXML
[08:25:02] info: Xml file was written to the provided writer.