Support configuring number of threads for parallel test execution

[cptartur]

Blocked by #901

[MaksymilianDemitraszek]

@cptartur Nobody is assigned here though it is in progress

[cptartur]

@cptartur Nobody is assigned here though it is in progress

It's not possible to assign people from outside of the organization who haven't commented on the issue.

[cptartur]

#888 did not resolve this. See #899 for details.

[cptartur]

Tokio's Builder doesn't seem to expose any config for limiting number of cores for blocking and non-blocking tasks at the same time. These are separate configurations and a non-zero amount of cores has to be assigned to both.

Perhaps once #901 is resolved, limiting just non-blocking threads will be enough for us.

[MaksymilianDemitraszek]

I would like to mention that current design of the architecure requires at least two threads so I would restrain from allowing single thread execution.

[Arcticae]

Closing because of the latter comment