[wasm][test] Compute more lengths automatically

Compute the length of more fields automatically, in particular names.

This is the fifth CL in a series to improve our module decoder tests and
make them more readable.

R=titzer@chromium.org

Bug: v8:8238
Change-Id: I1bd27f45380d82af2d7319f15ac7e37d5b9e4081
Reviewed-on: https://chromium-review.googlesource.com/c/1283077
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56721}

test/common/wasm/wasm-macro-gen.h

@@ -22,7 +22,6 @@
 #define TABLE_INDEX(v) U32V_1(v)
 #define EXCEPTION_INDEX(v) U32V_1(v)
 #define NO_NAME U32V_1(0)
-#define NAME_LENGTH(v) U32V_1(v)
 #define ENTRY_COUNT(v) U32V_1(v)
 
 #define ZERO_ALIGNMENT 0