regexp: warn when rx argument is not string or *regexp.Regexp

[mmorel-35]

assert.Regexp/assert.NotRegexp accept rx interface{}, but testify's matchRegexp only handles *regexp.Regexp and string correctly — anything else silently goes through regexp.MustCompile(fmt.Sprint(rx)), which produces unexpected results (e.g. []byte("pattern") compiles to its decimal byte representation).

Changes

• internal/checkers/regexp.go: Extended Check() to emit "use string or *regexp.Regexp as the first argument" when rx is neither a string type (typed, untyped, or string-underlying alias) nor *regexp.Regexp. Added isStringOrRegexpType helper.
• internal/testgen/gen_regexp.go: Added invalid test cases ([]byte arg) and valid test cases (pre-compiled *regexp.Regexp variable); updated template to declare compiledRegexp.

// Now flagged:
assert.Regexp(t, []byte(`\w+`), output) // regexp: use string or *regexp.Regexp as the first argument

// Valid — no warning:
assert.Regexp(t, `\w+`, output)
assert.Regexp(t, compiledRegexp, output)

Closes #81

[dolmen]

We must only accept:

• any *types.TypeParam
• *types.Basic that is strict string (BasicKind is String or UntypedString) (but NOT anything that is string underlying)
• any *types.Alias to above

The current implementation is too lax and doesn't handle TypeParam.

[ccoVeille]

Can you add the following to tests

• using 42 as an integer (typed or untyped, signed or not
• using float
• stringer

All of these rely on the fmt.Sprint that is done with non string or *regexp.Regexp

type foo struct{}

func (foo) String() string {
	return "42"
}

func TestFoo(t *testing.T) {
  assert.NotRegexp(t, false, "false")
	assert.NotRegexp(t, 42, "42")
	assert.NotRegexp(t, 42.0, "42")
	assert.NotRegexp(t, int8(42), "42")
	assert.NotRegexp(t, foo{}, "42")
}

All of these relies on something that would be better to avoid.

https://goplay.tools/snippet/Mmo4V36BHD1

Maybe something specific to stringer could be considered as a stringer can be rewritten to use .String()