vm: fix nested timeouts with inverse order + fix flaky test-vm-timeout

Checklist

• make -j4 test (UNIX) or vcbuild test nosign (Windows) passes
• a test and/or benchmark is included
• the commit message follows commit guidelines

Affected core subsystem(s)

vm, test

Description of change

Fix #6727 (closed):

• I think @trott is right in #6727 (closed) to assume the changed error message stems from the inner timeout in the test not checking whether it has witnessed its own timeout or the outer one.
• My best guess for why test-vm-timeout was flaky even before #6635 is that the outer timeout could interfere with handling of the inner timeout, e.g. during construction of the Error object that would be created. Increasing the outer timeout won’t have any adverse effects anyway.

/cc @bnoordhuis ?