editoast: bump axum-test from 15.3.0 to 15.6.0 in /editoast #10251

Job	Run time
check_railjson_generator	1m 49s
check_commits	5s
check_generated_railjson_sync	19s
build (core-build, core)	15s
check_integration_tests	1m 17s
build (editoast, editoast-test)	4m 31s
check_osrd_schema	45s
check_toml	5s
build (gateway-test, gateway-standalone, gateway-front)	1m 28s
Check final newline	17s
build (front-build, front-tests, front-devel, front-nginx)	4m 46s
build (osrdyne, osrdyne-test)	20s
check_infra_schema_sync	22s
check_editoast_lints	0s
check_editoast_openapi	0s
check_editoast_tests	0s
check_core	0s
check_front	0s
check_front_rtk_sync	0s
check_gateway	0s
check_osrdyne	0s
integration_tests	0s
	16m 19s

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