tests: expand testing of augmented assignment operators

[ee7]

Add some more tests for augmented assignment operators, and remove some existing ones that had similar coverage.

assignops.c4m was similar to basic10.c4m:

libcon4m/tests/basic10.c4m

Lines 1 to 15 in b4fdc8e

	"""
	Basic test of augmented assignment.
	"""
	"""
	$output:
	45
	"""
	x = 0
	for i in 0 to 10 {
	x += i
	}
	print(x)

The grammar specifies the augmented assignment operators here:

libcon4m/doc/reference.md

Lines 443 to 445 in b4fdc8e

	assign ::= expression assignmentOp expression EOS
	assignmentOp ::= ':' | '=' | '+=' | '-=' | '*=' | '/=' | '|=' | '&=' |
	'^=' | '>>=' | '<<='

Note that commit b4fdc8e changed the behavior of the /= operator so that:

• For division by zero, SIGFPE is no longer produced.
• For division by a negative number, the result is now correct.

Refs: #55
Refs: #57
Refs: #71