[feat] Drop tuple syntax for timer fields

[vkarak]

Fixes #1350.

[codecov-commenter]

Codecov Report

Merging #1470 into master will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1470      +/-   ##
==========================================
- Coverage   91.69%   91.67%   -0.02%     
==========================================
  Files          82       82              
  Lines       12852    12846       -6     
==========================================
- Hits        11784    11777       -7     
- Misses       1068     1069       +1     

Impacted Files	Coverage Δ
reframe/core/pipeline.py	92.80% <ø> (ø)
unittests/test_fields.py	97.54% <ø> (-0.03%)	⬇️
reframe/core/fields.py	98.95% <100.00%> (-0.05%)	⬇️
reframe/utility/typecheck.py	95.45% <0.00%> (-0.91%)	⬇️

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