[Process] Added process synchronization to the incremental output tests

The tests currently fail from time to time if the executing machine is under
heavy load. This leads to false negatives on Travis CI.

A side effect of the change is that the tests are much faster now.
fix shell command injection
add way to test command under windows
change command to which available under most unix systems

agree i will change it which is available under suse, ubuntu, debian, fedora, bsd
bug #11598 [Finder] Shell escape and windows support (Gordon Franke, gimler)

This PR was merged into the 2.3 branch.

Discussion
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[Finder] Shell escape and windows support

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT

Add escaping of command passed to Shell::testCommand().
Fix todo add support for windows.

Commits
-------

c70a226 change command to which available under most unix systems
85e77b1 add way to test command under windows
4fa9288 fix shell command injection
minor #11565 [Process] Added process synchronization to the incremental output tests (webmozart)

This PR was merged into the 2.3 branch.

Discussion
----------

[Process] Added process synchronization to the incremental output tests

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

The tests currently fail from time to time if the executing machine is under
heavy load. This leads to false negatives on Travis CI.

A side effect of the change is that the tests are much faster now.

Commits
-------

6dd3946 [Process] Added process synchronization to the incremental output tests