Bug 1373878 - part 1 - add --enable-rust-tests configure option; r=rillian

Because Rust tests require panic=unwind crates and therefore recompiling
all crates we normally use (since most of our crates use panic=abort),
we've elected to enable Rust tests only if the user asks for it.  Hence,
this configure option.

The configure option also enables build-time execution of the crates,
since it's not straightforward to determine at configure time the name
of the test binary Cargo will produce (Cargo produces test binaries of
the form ${NAME}-${HEX_FINGERPRINT}, but there's no way to determine
what ${HEX_FINGERPRINT} is without actually compiling the test binary).

build/moz.configure/rust.configure

@@ -240,3 +240,10 @@ set_config('RUSTC_COMMIT', depends(rustc_info)(lambda i: i.commit))
 # Until we remove all the other Rust checks in old-configure.
 add_old_configure_assignment('RUSTC', rustc)
 add_old_configure_assignment('RUST_TARGET', rust_target_triple)
+
+# This option is separate from --enable-tests because Rust tests are particularly
+# expensive in terms of compile time (especially for code in libxul).
+option('--enable-rust-tests',
+       help='Enable building of Rust tests, and build-time execution of them')
+
+set_config('MOZ_RUST_TESTS', depends('--enable-rust-tests')(lambda v: bool(v)))