tests: catch libkeymap round-trip regressions earlier

libkeymap already has good parser and dump coverage, but it lacked a
test for the invariant that matters when both sides evolve together:
a keymap should still describe the same canonical state after
parse -> dump -> parse.

Add a dedicated round-trip test over representative parser constructs,
including keymaps ranges, alt_is_meta, strings, compose rules,
modifiers, +rvalues and a real file-backed keymap. This protects the
canonical dump format against regressions that would not be caught by
parser-only or dump-only tests.

Signed-off-by: Alexey Gladkov <legion@kernel.org>