test: Split binary_expressions into separate tests

Tests should be as fine-grained as possible.
Not all the permutations were covered.
Not all the backends were actually generating anything useful, as the
functions were not called by an entry point.

Change-Id: I76658533d3112d202f7482a19531079550e66f83
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/58392
Auto-Submit: Ben Clayton <bclayton@google.com>
Commit-Queue: David Neto <dneto@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: David Neto <dneto@google.com>

test/expressions/binary/div/scalar-scalar/f32.wgsl

@@ -0,0 +1,6 @@
+[[stage(compute), workgroup_size(1)]]
+fn f() {
+    let a = 1.;
+    let b = 2.;
+    let r : f32 = a / b;
+}