test(infra): improved support for in-process testing

[jkowalski]

We currently don't have full picture of test coverage because some tests run kopia in out-of-process mode where code coverage is not collected.

This PR fixes this by making many more test cases runnable in-process. With this change almost all tests run in make test and make integration-tests is reduced to just a handful which will be moved next.

• support for killing of a running server using simulated Ctrl-C
• support for overriding os.Stdin
• support for passing environment variables to instances of Kopia in the same process - this is achieved by giving each instance of kopia their own environment variable prefix
• migrated many tests from the exe runner to in-process runner

[codecov]

Codecov Report

Merging #2169 (3558a2e) into master (a621cd3) will increase coverage by 0.86%.
The diff coverage is 96.55%.

@@            Coverage Diff             @@
##           master    #2169      +/-   ##
==========================================
+ Coverage   73.73%   74.59%   +0.86%     
==========================================
  Files         425      425              
  Lines       33006    33029      +23     
==========================================
+ Hits        24337    24639     +302     
+ Misses       6885     6607     -278     
+ Partials     1784     1783       -1     

Impacted Files	Coverage Δ
cli/command_snapshot_migrate.go	52.83% <0.00%> (ø)
cli/storage_providers.go	100.00% <ø> (ø)
cli/update_check.go	14.83% <0.00%> (+1.93%)	⬆️
cli/command_mount.go	20.98% <50.00%> (+0.98%)	⬆️
cli/config.go	76.81% <90.00%> (+0.24%)	⬆️
cli/app.go	80.39% <100.00%> (+1.39%)	⬆️
cli/command_diff.go	63.63% <100.00%> (ø)
cli/command_repository_change_password.go	66.66% <100.00%> (ø)
cli/command_repository_connect.go	91.78% <100.00%> (ø)
cli/command_repository_create.go	72.03% <100.00%> (ø)
... and 53 more

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