testscript: add 'unix' condition

From Go 1.19, the build constraint 'unix' proposed in golang/go#20322 is
satisfied by any sufficiently Unix-like value of GOOS, as defined by
src/go/build/syslist.go. This commit adds a predefined 'unix' condition
with the same meaning, available for use in test scripts. The condition
is satisfied if the target GOOS is one of the list of Unix-like systems
defined in 'imports.UnixOS'.

Fixes #166.

Co-authored-by: Daniel Martí <mvdan@mvdan.cc>

testscript/doc.go

@@ -108,6 +108,12 @@ should only run when the condition is satisfied. The predefined conditions are:
  - [gc] for whether Go was built with gc
  - [gccgo] for whether Go was built with gccgo
  - [go1.x] for whether the Go version is 1.x or later
+ - [unix] for whether the OS is Unix-like (that is, would match the 'unix' build
+   constraint)
+
+Any known values of GOOS and GOARCH can also be used as conditions. They will be
+satisfied if the target OS or architecture match the specified value. For example,
+the condition [darwin] is true if GOOS=darwin, and [amd64] is true if GOARCH=amd64.
 
 A condition can be negated: [!short] means to run the rest of the line
 when testing.Short() is false.

testscript/testdata/cond.txt

@@ -1,11 +1,23 @@
-[!exec:sh] skip 'sh not found in $PATH'
-
 # test that exactly one of gc and gccgo is set
-[gc] exec sh -c 'echo gc >> go-compiler'
-[gccgo] exec sh -c 'echo gccgo >> go-compiler'
-grep '\A(gc|gccgo)\n\z' go-compiler
+[gc] mkdir gc_true
+[gccgo] mkdir gccgo_true
+
+[gc] ! exists gccgo_true
+[!gc] exists gccgo_true
+[gccgo] ! exists gc_true
+[!gccgo] exists gc_true
 
 # test that go version build tags are set
-[go1.1] exec sh -c 'echo go1.1 >> go-version'
-[go2.1] exec sh -c 'echo go2.1 >> go-version'
-grep '\Ago1\.1\n\z' go-version
+[go1.1] mkdir go1.x
+[go2.1] mkdir go2.x
+
+exists go1.x
+! exists go2.x
+
+# unix should be true on Linux and MacOS, but not on Windows.
+# Both platforms are tested on CI.
+[unix] mkdir unix_true
+
+[linux] exists unix_true
+[darwin] exists unix_true
+[windows] ! exists unix_true

testscript/testscript.go

@@ -601,6 +601,8 @@ func (ts *TestScript) condition(cond string) (bool, error) {
 		return testenv.HasSymlink(), nil
 	case imports.KnownOS[cond]:
 		return cond == runtime.GOOS, nil
+	case cond == "unix":
+		return imports.UnixOS[runtime.GOOS], nil
 	case imports.KnownArch[cond]:
 		return cond == runtime.GOARCH, nil
 	case strings.HasPrefix(cond, "exec:"):