test(browser): Add integration test for changing transaction name in beforeSendTransaction

[Lms24]

This came up in an internal conversation: Some users need to update the transcation name in beforeSendTransaction. While we strictly recommend changing the root span/transaction name as early as possible, it should nevertheless be possible at the end of the root span lifecycle, too.

This test demonstrates that this works but it also shows "unexpected" behaviour: Despite users setting an explicit source in beforeSendTransaction, we overwrite the source to 'custom'. There's good reason for this in general: Changing the name of something the SDK previously identified as source url or route should be reflected by the source. But we don't account for the case where users explicitly set the source.

For the moment though, this not an actual problem because sources route and custom are treated equally by Relay.

[codecov]

❌ 1 Tests Failed:

Tests completed	Failed	Passed	Skipped
663	1	662	24

View the top 1 failed tests by shortest run time

tracing/browserTracingIntegration/pageload-update-txn-name/test.ts sets the source to custom when updating the transaction name

Stack Traces | 30s run time

test.ts:13:11 sets the source to custom when updating the transaction name

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