Improve code coverage of Rust testing

I enabled code coverage and ran the gdb test suite, and noticed that
the new Rust parser was missing testing on a few lines that were easy
to cover.  This patch mostly adds tests for certain syntax errors; but
this process also uncovered a couple of real bugs: I must have
cut-and-pasted the 'sizeof' parsing code from some other code, because
it is checking for KW_MUT (the old bison parser did not do this), and
the array length check is actually impossible because a negative
number like '-1' is parsed as two tokens.

gdb/ChangeLog
2021-04-22  Tom Tromey  <tom@tromey.com>

	* rust-parse.c (rust_parser::parse_sizeof): Remove KW_MUT code.
	(struct typed_val_int) <val>: Now ULONGEST.
	(rust_parser::parse_array_type): Remove negative check.
	(rust_lex_int_test): Change 'value' to ULONGEST.

gdb/testsuite/ChangeLog
2021-04-22  Tom Tromey  <tom@tromey.com>

	* gdb.rust/modules.exp: Add checks for syntax errors.
	* gdb.rust/expr.exp: Add checks for syntax errors.
	* gdb.rust/simple.exp: Add checks for syntax errors.

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