Fix hang after multiple precompiled browser tests

[natebosch]

Fixes #2294

Avoid creating extra unexpected BrowserManager instances by caching the
future in the _browserManagers map without any async delay. Previously
it was possible for two managers to be created if the second suite is
loaded before the first suite's compilerSupport was resolved. This was
not a problem for tests that get compiled by the test runner because the
compilation would delay the second suite load until after the first
suite's compilerSupport has resolved. It is not a problem when running
without concurrency because that delays the second suite load.

Add a concurrency argument to the regression test, otherwise the default
is to run with concurrency 1 which works around the bug.

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PR Health

Changelog Entry ✔️

Package	Changed Files

Changes to files need to be accounted for in their respective changelogs.

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Package publishing

Package	Version	Status	Publish tag (post-merge)
package:test	1.25.12	ready to publish	test-v1.25.12
package:test_core	0.6.8	ready to publish	test_core-v0.6.8
package:checks	0.3.1-wip	WIP (no publish necessary)
package:fake_async	1.3.2	already published at pub.dev
package:matcher	0.12.18-wip	WIP (no publish necessary)
package:test_api	0.7.4	already published at pub.dev

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