Clean up GenerateBreakTest (#975)

This fulfils the following action item:
UTC-155-A89 Robin Leroy, PAG Document extra classes used for testing characters in the segmentation test HTML files for 11.0. [E.g. ZWJ_FE, CM1_CM, etc.] (Retargeted for 13.0, 14.0, 15.0.)

It also fixes #354.

It also changes the pair table in LineBreakTest.html to show the three way direct/indirect/prohibited break distinction (across spaces), like the old pair table in UAX14 (see https://www.unicode.org/notes/tn54/alba-2.html?v=9.0.0).

As in #970, the test files are not diffable, but I tested them with ICU.

I tried improving the stability of the sample generation a little bit, but it is not as fancy as what Mark suggests in https://www.unicode.org/notes/tn54/alba-2.html?v=9.0.0#p478. I might do that in another PR.

Author: eggrobin