pkg/cover: random failures of TestCover

[a-nogikh]

It's been observed several times over the last days.

2025-04-17T08:04:19.3806494Z --- FAIL: TestCover (0.07s)
2025-04-17T08:04:19.3806893Z     --- FAIL: TestCover/64_fork (4.12s)
2025-04-17T08:04:19.3807154Z         --- FAIL: TestCover/64_fork/11 (0.12s)
2025-04-17T08:04:19.3807610Z             run_test.go:417: program execution failed: status=ExecFailure err=context closed while waiting the result
2025-04-17T08:04:19.3830099Z ##[error]program execution failed: status=ExecFailure err=context closed while waiting the result
2025-04-17T08:04:19.3837620Z             run_test.go:523: coverage is not supported: call 0 failed with errno 998
2025-04-17T08:04:19.3838634Z ##[error]coverage is not supported: call 0 failed with errno 998

[a-nogikh]

The first message is somewhat misleading

2025-04-17T08:04:19.3807610Z             run_test.go:417: program execution failed: status=ExecFailure err=context closed while waiting the result
2025-04-17T08:04:19.3830099Z ##[error]program execution failed: status=ExecFailure err=context closed while waiting the result

It's a consequence rather than the reason. The actual reason is that rpcserver.RunLocal() fails with

 2025-04-17T08:04:19.3837620Z             run_test.go:523: coverage is not supported: call 0 failed with errno 998

In other cases I've also observed

            run_test.go:523: coverage is not supported: too many calls in output. slot=64 (errno 9: Bad file descriptor). . process exited with status 67.

syzkaller/pkg/runtest/run_test.go

Lines 516 to 518 in 9882047

	err := rpcserver.RunLocal(ctx, cfg)
	done()
	errc <- err