Support string id field

[tom-tan]

It partially solves #110 by supporting id field with string value.

Currently cwltest copies label field to short_name field and print it for each test.
This request decides short_name as follows:

• use label field if available
• use id field if it has a string value
• otherwise cwltest does not set short_name as same as before this request

---

• Before merging this request (See tests/conditionals/test-index.yaml#L419 in cwl-v1.2):

$ ./run_test.sh -l
...
[306] Default inputs, choose step to run based on what was provided, first case
[307] Default inputs, choose step to run based on what was provided, second case
...

• After merging this request:

$ ./run_test.sh -l
...
[306] cond-with-defaults-1: Default inputs, choose step to run based on what was provided, first case
[307] cond-with-defaults-2: Default inputs, choose step to run based on what was provided, second case
...

[codecov]

Codecov Report

Merging #137 (6b254df) into main (9dac068) will increase coverage by 0.48%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #137      +/-   ##
==========================================
+ Coverage   56.94%   57.43%   +0.48%     
==========================================
  Files           2        2              
  Lines         439      444       +5     
  Branches      128      130       +2     
==========================================
+ Hits          250      255       +5     
  Misses        146      146              
  Partials       43       43              

Impacted Files	Coverage Δ
cwltest/__init__.py	60.61% <100.00%> (+0.61%)	⬆️

---

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[tom-tan]

Thanks!