pkg: avoid log.Fatal calls

[a-nogikh]

These calls prevent graceful error handling in the caller code. We should just return error and let individual tools decide what to do.

[a-nogikh]

@dvyukov This PR has so many little changes to the tricky parts of the code that there's a high risk that I introduced some new concurrency/synchronization bugs to syzkaller. The PR needs a careful code review.

If there's any way to do those changes in a simpler way, please let me know.

[a-nogikh]

On my workstation it fails specifically on the recently added align3 test

$ ./tools/syz-env go test  ./pkg/runtest/ -run TestUnit/32_fork -v 
gcr.io/syzkaller/env:latest
=== RUN   TestUnit
=== RUN   TestUnit/32_fork
=== PAUSE TestUnit/32_fork
=== CONT  TestUnit/32_fork
    run.go:77: align0 none                           : OK
    run.go:77: align0 none/thr                       : OK
    run.go:77: align0_be none                        : SKIP (excluded by constraints)
    run.go:77: align0_be none/thr                    : SKIP (excluded by constraints)
    run.go:77: align3 none                           : FAIL: run 0: wrong call 0 result 9, want 0
    run.go:77: align3 none/thr                       : FAIL: run 0: wrong call 0 result 9, want 0
    run.go:77: bf none                               : OK
    run.go:77: bf none/thr                           : OK
< ... >
    run.go:77: zero_args none                        : OK
    run.go:77: zero_args none/thr                    : OK
    run.go:77: ok: 29, broken: 0, skip: 11, fail: 2
    run_test.go:121: tests failed
--- FAIL: TestUnit (0.00s)
    --- FAIL: TestUnit/32_fork (6.05s)
FAIL
FAIL    github.com/google/syzkaller/pkg/runtest 14.689s

[a-nogikh]

Ah, okay, the test fails on the upstream code as well -- we set -short on our CI and the align3 program is never run there.

[a-nogikh]

PTAL