test: enabled_if restriction within executable stanza (#14848)

This commit adds a cram test from the reproducer in #14789 to
demonstrate the restricted set of allowed_vars in enabled_if used within
the exectuable stanza.

Author: Alizter

test/blackbox-tests/test-cases/enabled_if/enabled_if-exec-read.t/config/disabled

@@ -0,0 +1 @@
+true

test/blackbox-tests/test-cases/enabled_if/enabled_if-exec-read.t/config/enable_executable

@@ -0,0 +1 @@
+let () = print_endline "should be disabled!"

test/blackbox-tests/test-cases/enabled_if/enabled_if-exec-read.t/disabled.ml

@@ -0,0 +1,9 @@
+(executable
+ (name main)
+ ;; The config file is in a subdirectory to avoid the spurious cycles in
+ ;; enabled_if. See the enabled_if/eif-library-cycle.t test for instance.
+ (enabled_if (= %{read:./config/enable_executable} "true")))
+
+(executable
+ (name disabled)
+ (enabled_if (= %{read:./config/disabled} "true")))

test/blackbox-tests/test-cases/enabled_if/enabled_if-exec-read.t/dune

@@ -0,0 +1 @@
+let () = print_endline "hello"

test/blackbox-tests/test-cases/enabled_if/enabled_if-exec-read.t/main.ml

@@ -0,0 +1,48 @@
+Test that `enabled_if` fields work as expected for executables with read pform.
+
+Check that enabled_if doesn't work before dune lang 2.3
+
+  $ make_dune_project 2.0
+  $ dune exec ./main.exe
+  File "dune", line 5, characters 1-59:
+  5 |  (enabled_if (= %{read:./config/enable_executable} "true")))
+       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+  Error: 'enabled_if' is only available since version 2.3 of the dune language.
+  Please update your dune-project file to have (lang dune 2.3).
+  [1]
+
+Check that only common vars are allowed in enabled_if within the executable
+stanza in older dune lang versions:
+
+  $ make_dune_project 3.0
+  $ dune exec ./main.exe
+  File "dune", line 5, characters 16-50:
+  5 |  (enabled_if (= %{read:./config/enable_executable} "true")))
+                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+  Error: Only architecture, system, model, os_type, ccomp_type, profile,
+  ocaml_version, context_name, arch_sixtyfour and env variables are allowed in
+  this 'enabled_if' field. Please upgrade your dune language to at least 3.15.
+  [1]
+
+Check that the executable is correctly enabled:
+
+  $ make_dune_project 3.24
+  $ dune exec ./main.exe
+  File "dune", line 5, characters 16-50:
+  5 |  (enabled_if (= %{read:./config/enable_executable} "true")))
+                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+  Error: Only architecture, system, model, os_type, ccomp_type, profile,
+  ocaml_version, context_name, arch_sixtyfour and env variables are allowed in
+  this 'enabled_if' field. Please upgrade your dune language to at least 3.15.
+  [1]
+
+Check that the executable is correctly disabled:
+
+  $ dune exec ./disabled.exe
+  File "dune", line 5, characters 16-50:
+  5 |  (enabled_if (= %{read:./config/enable_executable} "true")))
+                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+  Error: Only architecture, system, model, os_type, ccomp_type, profile,
+  ocaml_version, context_name, arch_sixtyfour and env variables are allowed in
+  this 'enabled_if' field. Please upgrade your dune language to at least 3.15.
+  [1]