Auto merge of #2670 - matklad:simplify-tests, r=alexcrichton

Simplify tests

@alexcrichton I think there is one bit of tests that could be simplified.

What do you think about writing this

```Rust
test!(simple {
    pkg("foo", "0.0.1");

    assert_that(cargo_process("install").arg("foo"),
                execs().with_status(0).with_stdout(&format!("\
[UPDATING] registry `[..]`
[DOWNLOADING] foo v0.0.1 (registry file://[..])
[COMPILING] foo v0.0.1 (registry file://[..])
[INSTALLING] {home}[..]bin[..]foo[..]
",
        home = cargo_home().display())));
    assert_that(cargo_home(), has_installed_exe("foo"));

    assert_that(cargo_process("uninstall").arg("foo"),
                execs().with_status(0).with_stdout(&format!("\
[REMOVING] {home}[..]bin[..]foo[..]
",
        home = cargo_home().display())));
    assert_that(cargo_home(), is_not(has_installed_exe("foo")));
});
```

instead of this

```Rust
test!(simple {
    pkg("foo", "0.0.1");

    assert_that(cargo_process("install").arg("foo"),
                execs().with_status(0).with_stdout(&format!("\
{updating} registry `[..]`
{downloading} foo v0.0.1 (registry file://[..])
{compiling} foo v0.0.1 (registry file://[..])
{installing} {home}[..]bin[..]foo[..]
",
        updating = UPDATING,
        downloading = DOWNLOADING,
        compiling = COMPILING,
        installing = INSTALLING,
        home = cargo_home().display())));
    assert_that(cargo_home(), has_installed_exe("foo"));

    assert_that(cargo_process("uninstall").arg("foo"),
                execs().with_status(0).with_stdout(&format!("\
{removing} {home}[..]bin[..]foo[..]
",
        removing = REMOVING,
        home = cargo_home().display())));
    assert_that(cargo_home(), is_not(has_installed_exe("foo")));
});
```

This PR has a proof of concept implementation of this feature applied to a couple of tests.

r? @alexcrichton