Because your second regexp primarily uses a negative lookahead, you can remodel that slightly and stick it right at the beginning of the other expression. First, I'm going to change your second regex to:
"(?!.*(?:Test|test|TEST))"
In english, the string may not contain any number of (or zero) characters followed by test.
Then, I'm going to stick that right at the beginning of your other expression
passwordStrengthRegularExpression="^(?!.*(?:Test|test|TEST))(?=.*\d)(?=.*[a-z])(?=.*[A-Z])(?=.*[@#$%]).{6,20}$"
Finally, I'm going to show you how to make only one part of a regex case-insensitive. This may or may not be supported depending on what program this is actually for.
passwordStrengthRegularExpression="^(?!.*(?i:test))(?=.*\d)(?=.*[a-z])(?=.*[A-Z])(?=.*[@#$%]).{6,20}$"
See the (?i:...)
? That means that the flags between the ?
and the :
are applied only to that part of the expression, that is, only that area is case-insensitive.