patch 9.2.0783: tests: personal spell files leak into later tests

patch 9.2.0783: tests: personal spell files leak into later tests

Problem:  Test_spelldump_prefixtree_overflow() resets 'runtimepath' to
          defaults, which can expose personal spell files to later tests
          (Jun-ichi Takimoto, after v9.2.0662)
Solution: Save and restore the test runner's 'runtimepath' value (coyaSONG).

fixes:  #20765
closes: #20766

Co-Authored-By: OpenAI Codex <noreply@openai.com>
Signed-off-by: coyaSONG <66289470+coyaSONG@users.noreply.github.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>