SA1030: actually guard some tests behind Go 1.15

We need actual Go build tags to prevent compiling test cases with an older
Go version.

We also need to actually remove the offending lines of code from the
more general test case.

While we're here, we can also rename the test directory, 1.15 != 1.5.

Author: dominikh

staticcheck/lint_test.go

@@ -43,7 +43,7 @@ func TestAll(t *testing.T) {
 		"SA1029": {{Dir: "CheckWithValueKey"}},
 		"SA1030": {
 			{Dir: "CheckStrconv"},
-			{Dir: "CheckStrconv_go15", Version: "1.15"},
+			{Dir: "CheckStrconv_go115", Version: "1.15"},
 		},
 		"SA2000": {{Dir: "CheckWaitgroupAdd"}},
 		"SA2001": {{Dir: "CheckEmptyCriticalSection"}},

staticcheck/testdata/src/CheckStrconv/CheckStrconv.go

@@ -3,11 +3,6 @@ package pkg
 import "strconv"
 
 func fn() {
-	strconv.ParseComplex("", 32) // want `'bitSize' argument is invalid, must be either 64 or 128`
-	strconv.ParseComplex("", 64)
-	strconv.ParseComplex("", 128)
-	strconv.ParseComplex("", 256) // want `'bitSize' argument is invalid, must be either 64 or 128`
-
 	strconv.ParseFloat("", 16) // want `'bitSize' argument is invalid, must be either 32 or 64`
 	strconv.ParseFloat("", 32)
 	strconv.ParseFloat("", 64)
@@ -37,12 +32,6 @@ func fn() {
 	strconv.ParseUint("", 36, 0)
 	strconv.ParseUint("", 37, 0) // want `'base' must not be larger than 36`
 
-	strconv.FormatComplex(0, 'e', 0, 32) // want `'bitSize' argument is invalid, must be either 64 or 128`
-	strconv.FormatComplex(0, 'e', 0, 64)
-	strconv.FormatComplex(0, 'e', 0, 128)
-	strconv.FormatComplex(0, 'e', 0, 256) // want `'bitSize' argument is invalid, must be either 64 or 128`
-	strconv.FormatComplex(0, 'j', 0, 64)  // want `'fmt' argument is invalid: unknown format 'j'`
-
 	strconv.FormatFloat(0, 'e', 0, 18) // want `'bitSize' argument is invalid, must be either 32 or 64`
 	strconv.FormatFloat(0, 'e', 0, 32)
 	strconv.FormatFloat(0, 'e', 0, 64)

staticcheck/testdata/src/CheckStrconv_go15/CheckStrconv.go renamed to staticcheck/testdata/src/CheckStrconv_go115/CheckStrconv.go

@@ -1,3 +1,5 @@
+// +build go1.15
+
 package pkg
 
 import "strconv"

staticcheck/testdata/src/CheckStrconv_go115/stub.go

@@ -0,0 +1 @@
+package pkg