Enable modifying <crate>_ROOT_TEST variable.

README.md

@@ -190,6 +190,12 @@ Default: Determined with existing file (`main.rs` or `lib.rs`) in crate dir.
 
 Specify the crate root file (entry file when compiling the crate).
 
+### `<crate>_ROOT_TEST` ###
+
+Default: Crate root file
+
+Specify the crate root file used for tests.
+
 ### `<crate>_TYPE` ###
 
 Default: Determined with crate root (`main.rs` or `lib.rs`).

rust.mk

@@ -120,7 +120,7 @@ $$(error Unknown crate type '$$($(1)_TYPE)' for '$(1)')
 endif
 
 ### Crate common variables (after type resolving)
-$(1)_ROOT_TEST          =   $$($(1)_ROOT)
+$(1)_ROOT_TEST          ?=  $$($(1)_ROOT)
 $(1)_NAMES              =   $$(addprefix $$($(1)_PREFIX),$$(shell $$(RUSTC) $$(RUSTCFLAGS) $$($(1)_RUSTCFLAGS) --crate-file-name $$($(1)_ROOT)))
 $(1)_NAME               =   $$(firstword $$($(1)_NAMES))