Enable usetesting linter and simplify tests

[alexandear]

usetesting tells that we can replace os.Setenv with t.Setenv and os.MkdirTemp with t.TempDir. As a result, this slightly simplifies tests.

$ golangci-lint run
pkg/dbmate/db_test.go:112:14: os.MkdirTemp() could be replaced by t.TempDir() in TestDumpSchema (usetesting)
        dir, err := os.MkdirTemp("", "dbmate")
                    ^
pkg/dbmate/db_test.go:146:14: os.MkdirTemp() could be replaced by t.TempDir() in TestAutoDumpSchema (usetesting)
        dir, err := os.MkdirTemp("", "dbmate")
                    ^
pkg/dbmate/db_test.go:190:14: os.MkdirTemp() could be replaced by t.TempDir() in TestLoadSchema (usetesting)
        dir, err := os.MkdirTemp("", "dbmate")
                    ^
pkg/dbmate/db_test.go:559:15: os.MkdirTemp() could be replaced by t.TempDir() in TestFindMigrationsAbsolute (usetesting)
                dir, err := os.MkdirTemp("", "dbmate")
                            ^
main_test.go:15:21: os.Setenv() could be replaced by t.Setenv() in TestGetDatabaseUrl (usetesting)
        require.NoError(t, os.Setenv("DATABASE_URL", "foo://example.org/one"))
                           ^
main_test.go:16:21: os.Setenv() could be replaced by t.Setenv() in TestGetDatabaseUrl (usetesting)
        require.NoError(t, os.Setenv("CUSTOM_URL", "foo://example.org/two"))
                           ^
main_test.go:88:5: os.Setenv() could be replaced by t.Setenv() in TestLoadEnvFiles (usetesting)
                                os.Setenv(pair[0], pair[1])
                                ^
main_test.go:134:3: os.Setenv() could be replaced by t.Setenv() in TestLoadEnvFiles (usetesting)
                os.Setenv("FIRST", "not one")
                ^

[dossy]

Makes sense, looks good, and tests still pass.