`swift build --build-tests` doesn't work with `--enable-test-discovery` flag

[MaxDesiatov]

It appears the lack of tests hurt us here: this pull broke swift build --build-tests registering the --enable-test-discovery flag also. My Android CI builds swift-numerics trunk daily with the latest Swift trunk snapshot builds and those flags: it now errors with the March 1 snapshot.

I can reproduce the crash on linux x86_64 with the following commands, which work with the last Feb. 29 trunk snapshot before this pull:

> ../swift-DEVELOPMENT-SNAPSHOT-2024-03-01-a-ubi9/usr/bin/swift build --build-tests --enable-test-discovery
> .build/x86_64-unknown-linux-gnu/debug/swift-numericsPackageTests.xctest

Originally posted by @finagolfin in #7377 (comment)

[grynspan]

--enable-test-discovery is deprecated and has no effect anymore; it will be removed in a future update. We should move this over to swift-numerics to remove the LinuxMain.swift file. I'm guessing WindowsMain.swift can be updated too.

@MaxDesiatov I don't seem to be able to move the issue over myself (presumably a permissions issue.)

[MaxDesiatov]

I can't move that either. Created apple/swift-numerics#280 in the meantime.

[grynspan]

Reopening while I make sure that --enable-test-discovery doesn't outright break the build. It'll either end up being a one-line fix or a complete removal of the option (save for a vestigial deprecation warning.)