I followed-up on this in Testing on Android - #7 by marcprux, but for the record, it was pretty straightforward to do this:
swift build --build-tests --swift-sdk <triple> -Xlinker -shared -Xlinker -no-pie
…and the resulting .xctest binary is a shared object rather than a command-line executable:
$ file ./.build/aarch64-unknown-linux-android28/debug/SwiftTestingDemoPackageTests.xctest
SwiftTestingDemoPackageTests.xctest: ELF 64-bit LSB shared object, ARM aarch64, version 1 (SYSV), dynamically linked, BuildID[sha1]=5b10a416063772297352c452f1bdc41d032a916b, with debug_info, not stripped