Move subplot tests to a feature so subplot doesn't affect most builds

Installing sq from crates.io (cargo install sequoia-sq) was broken by a
semver-compatible change in Tera.  Running cargo test uses the lockfile
and isn't affected.

This has the side benefit of reducing dependency bloat, the baseline
depends on check/build/build --release but in the case of a non-release
build the dependency count goes from 403 to 315.

Fixes #2.

The subplot/tera issue was likely triggered by this change in tera
1.18: <https://github.com/Keats/tera/pull/799>.

.gitlab-ci.yml

@@ -53,7 +53,7 @@ bookworm:
   interruptible: true
   image: 192.168.122.1:5000/sequoia-pgp/build-docker-image/bookworm:latest
   script:
-    - cargo test
+    - cargo test --features=subplot
     - if ! git diff --quiet Cargo.lock ; then echo "Cargo.lock changed.  Please add the change to the corresponding commit." ; git diff ; false ; fi
     - if ! git diff --quiet ; then echo "The build changed the source.  Please investigate." ; git diff ; fi
   variables:
@@ -136,7 +136,7 @@ rust-stable:
     - rustup override set stable
     - *before_script_end
   script:
-    - cargo test
+    - cargo test --features=subplot
   variables:
     CARGO_TARGET_DIR: /target
     CARGO_HOME: /cargo
@@ -181,7 +181,7 @@ windows-gnu-cng:
   script:
     # https://github.com/rust-lang/cargo/issues/5015
     - pacman --noconfirm --needed -S mingw-w64-x86_64-openssl
-    - cargo test --tests --no-default-features --features crypto-cng,compression-bzip2
+    - cargo test --tests --no-default-features --features crypto-cng,compression-bzip2,subplot
   after_script: [] # scriptlet doesn't work on Powershell
   variables:
     CFLAGS: "" # Silence some C warnings when compiling under Windows

Cargo.toml

@@ -61,7 +61,7 @@ clap_mangen = "0.2"
 chrono = "0.4.10"
 sequoia-openpgp = { version = "1.13", default-features = false }
 sequoia-net = { version = "0.27", default-features = false }
-subplot-build = "0.6.0"
+subplot-build = { version = "0.6.0", optional = true }
 cfg-if = "1"
 
 [dev-dependencies]
@@ -90,6 +90,7 @@ crypto-cng = ["sequoia-openpgp/crypto-cng"]
 crypto-rust = ["sequoia-openpgp/crypto-rust"]
 compression-bzip2 = ["sequoia-openpgp/compression-bzip2"]
 autocrypt = ["sequoia-autocrypt"]
+subplot = ["subplot-build"]
 
 [profile.release]
 debug = true

build.rs

@@ -13,6 +13,7 @@ fn main() {
     println!("cargo:rerun-if-changed=build.rs");
 
     // Generate subplot tests.
+    #[cfg(feature = "subplot")]
     subplot_build::codegen(Path::new("sq.subplot"))
         .expect("failed to generate code with Subplot");
 

tests/sq-subplot.rs

@@ -1 +1,2 @@
+#![cfg(feature = "subplot")]
 include!(concat!(env!("OUT_DIR"), "/sq.rs"));