tests, meson_options: Add test-dev-install option

As the test-plugin.so can be seen as back-door into the compositor
internals, only expose it as a build time feature, but still making it
easier for people to use the test harness in case they would like to do
so. Add a short explanation into README to inform users of that.

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
5 jobs for wip/mvlad/test-harness in 4 minutes and 42 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Container Prep
passed #9151018
container_prep

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  Build
passed #9151020
kvm
build-docs

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passed #9151019
kvm
build-native-meson-default-options

00:02:52

passed #9151021
kvm
build-native-meson-no-gl-renderer

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  Pages
passed #9151022
docs-and-coverage

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