Move the tests of unused import removal into case files

RemoveUnusedImportsTest was a table inherited from google-java-format:
each row an array of input lines and one of output lines, joined with
\n and run through RemoveUnusedImports alone. 19 of its 23 rows are now
cases under test-cases/imports, one file each, named after what it
checks, and so is the one test of RemoveUnusedImportsCaseLabelsTest. A
case runs the whole formatter, so its after block is the row's
expected output, formatted: for every row the formatted input and the
formatted expectation are the same text, and a second run changes
nothing. One row's input was formatted already, so its case lays the
same code out another way and has something to format.

Four rows stay in RemoveUnusedImportsTest, as plain tests with text
blocks. The three of #37 cannot be cases: a case fixes the imports
before it formats, and formatting takes a doubled blank line away by
itself, while the command line formats first. The module import parses
from JDK 23 on, and the cases run on JDK 21 as well.

With the RemoveUnusedImports of 21.X.X, the same four cases fail as
the rows did: the ones with a used import from java.lang.

Tests on JDK 21: FormatterCasesTest 887 -> 907, RemoveUnusedImportsTest
23 -> 4 (the module import skipped), RemoveUnusedImportsCaseLabelsTest
1 -> gone; 1413 tests in the module, as before. On JDK 25 the four
tests of RemoveUnusedImportsTest all run, and the 907 cases pass.

Author: abashev