[analyzer] SATest: Weaken assumption about HTML files

Instead of assuming there is an HTML file for each diagnostics, consider
the HTML files only when they exist, individually of each other.

After generating the reference data, running

  python /scripts/SATest.py build --projects simbody

was resulting in this error:

    File "/scripts/CmpRuns.py", line 250, in read_single_file
      assert len(d['HTMLDiagnostics_files']) == 1
  KeyError: 'HTMLDiagnostics_files'

Reviewed By: steakhal

Differential Revision: https://reviews.llvm.org/D126197