[feat] Simplify custom user imports in tests

[vkarak]

This PR introduces three changes:

1. The directory of a test is temporarily added to sys.path during loading of the test. This allows to directly import modules defined below the test directory.
2. The loader changes its working directory temporarily to the test's directory while loading.
3. Two utility functions are added to simplify loading of user modules.

(2) and (3) allow users to import modules with a single call to the library function (see tutorial example).

Closes #2595.

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (develop@b992afd). Click here to learn what that means.
Patch coverage: 94.28% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             develop    #2801   +/-   ##
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  Coverage           ?   86.80%           
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  Files              ?       60           
  Lines              ?    11395           
  Branches           ?        0           
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  Hits               ?     9891           
  Misses             ?     1504           
  Partials           ?        0           

Impacted Files	Coverage Δ
reframe/frontend/loader.py	94.87% <89.47%> (ø)
reframe/utility/__init__.py	93.39% <100.00%> (ø)

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