tests: adjust the project and load path to add test dependencies in test_module

[benlorenz]

To have test dependencies (e.g. Documenter or Aqua) available when running tests via test_module.
Achieved by creating a temporary project using the test/Project.toml file and stacking this with the currently active project.
An alternative would be to try to hook into some of the internals of Pkg.jl and reuse that but it seemed rather fragile.

Seems to work fine in local tests, both with new=false and new=true, even for the Aqua.jl tests which use several temporary environments itself. Tested with the parallel test jobs as well.

This feature is always active unless one specifies tempproject=false, which is used in the main runtests.jl to avoid creating new projects for every testfile.

fixes #3354

[codecov]

Codecov Report

Merging #3357 (d3a0de7) into master (4b20afe) will decrease coverage by 0.04%.
Report is 3 commits behind head on master.
The diff coverage is 43.47%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3357      +/-   ##
==========================================
- Coverage   81.84%   81.81%   -0.04%     
==========================================
  Files         556      556              
  Lines       74361    74399      +38     
==========================================
+ Hits        60864    60870       +6     
- Misses      13497    13529      +32     

Files	Coverage Δ
src/utils/tests.jl	61.53% <43.47%> (-6.24%)	⬇️

... and 7 files with indirect coverage changes

[thofma]

I tested it with Groups and some other failing examples in the experimental folder. Seems to work, thanks!

[thofma]

Out of curiosity, why add the try here?

[benlorenz]

For the finally block that restores the load path and active project (when exiting the block with an exception).