[WebAssembly] Rename legacy EH tests (#107166)

Give each test in `cfg-stackify-eh-legacy.ll` a name rather than
something like `test5`, because I plan to copy many of these test into a
new file that tests for the new EH (exnref) and some of the tests here
are not applicable to the new EH so the numbering will be different,
which can make things confusing.

Also this removes `test_` prefixes in the test function names in
`exception-legacy.ll`, because, well, we all know they are tests.

Author: aheejin