tests: Compile in advance

Cargo is always trying to compile things if they are not ready yet. It caused the first test to take a lot of time, and potentially time out.

src/meson.build

@@ -62,10 +62,26 @@ rslibs = custom_target(
     command: [cargo_script, '@OUTPUT@', 'build']
 )
 
+build_rstests = custom_target(
+    'build_rstests',
+    build_by_default: false,
+    # HACK: this target needs to build before all the tests,
+    # but it doesn't produce anything stable.
+    # Declaring build_by_default with some random but irrelevant output
+    # ensures that it's always built as it should
+    build_always_stale: true,
+    output: ['src'],
+    install: false,
+    console: true,
+    command: [cargo_script, '', 'build', '--tests'],
+    depends: rslibs, # no point building tests if the code itself fails
+)
+
 test(
     'rstest',
     cargo_script,
-    args: ['', 'test']
+    args: ['', 'test'],
+    depends: build_rstests,
 )
 
 libsqueekboard = static_library('libsqueekboard',