Do not inject test harness for --cfg test

[sanxiyn]

Fix #33670.

[rust-highfive]

r? @nikomatsakis

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[sanxiyn]

config field of TestCtxt is dead. dead_code lint actually detects it, but test.rs disables this lint with allow attribute.

cfg argument of modify_for_testing is ultimately passed to ExtCtxt::new. In turn, the only use in ExtCtxt is passing to Parser::new so that syntax extensions can use the parser. test.rs only uses builder, not parser.

[nikomatsakis]

@bors r+

[bors]

📌 Commit 3c4eb01 has been approved by nikomatsakis

[nikomatsakis]

@Manishearth I did not think this was plugin breaking but it occurs to me I do not know for sure. :)

[Manishearth]

Technically is, by the most stringent definition, but it doesn't look like it will break anything

[bors]

⌛ Testing commit 3c4eb01 with merge 623af22...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-32cross-opt, auto-linux-64-cargotest, auto-linux-64-cross-armhf, auto-linux-64-cross-armsf, auto-linux-64-cross-freebsd, auto-linux-64-cross-netbsd, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-opt-mir, auto-linux-64-opt-rustbuild, auto-linux-64-x-android-t, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-mac-64-opt-rustbuild, auto-mac-cross-ios-opt, auto-win-gnu-32-nopt-t, auto-win-gnu-32-opt, auto-win-gnu-32-opt-rustbuild, auto-win-gnu-64-nopt-t, auto-win-gnu-64-opt, auto-win-msvc-32-cross-opt, auto-win-msvc-32-opt, auto-win-msvc-64-cargotest, auto-win-msvc-64-opt, auto-win-msvc-64-opt-mir, auto-win-msvc-64-opt-rustbuild