Clean up and reformat HF26 tests and datagen_tests

Style of HF26 and datagen tests was inconsistent with other tests. These changes removes inconsistencies.

In both tests

• sort imports
• remove spaces from default parameters: a = 1 -> a=1
• separate tests with 2 empty lines

In datagen tests

• remove unneeded try-excepts
• add type annotations

In HF26 tests

• short too long lines
• remove spaces from method calls, within parentheses: foo( 'a' ) -> foo('a')
• replace "strings" with 'strings' (single quotes instead of double)