makefile: run Rust tests if cargo is installed

While no particular minimum toolchain version is targeted as of yet, this
serves as a first step to make more people/machines run the Rust tests.

Makefile

@@ -16,6 +16,7 b' export LANGUAGE=C'
 export LC_ALL=C
 TESTFLAGS ?= $(shell echo $$HGTESTFLAGS)
 OSXVERSIONFLAGS ?= $(shell echo $$OSXVERSIONFLAGS)
+CARGO = cargo
 
 # Set this to e.g. "mingw32" to use a non-default compiler.
 COMPILER=
@@ -110,6 +111,10 b' dist-notests:\tdoc MANIFEST.in'
 check: tests
 
 tests:
+        # Run Rust tests if cargo is installed
+	if command -v $(CARGO) >/dev/null 2>&1; then \
+		cd $(HGROOT)/rust/hg-cpython && $(CARGO) test --quiet --all; \
+	fi
 	cd tests && $(PYTHON) run-tests.py $(TESTFLAGS)
 
 test-%: