Fix some characterization tests

A few changes had cropped up with `_NIX_TEST_ACCEPT=1`:

1. Blake hashing test JSON had a different indentation

2. Store URI had improper non-quoted spaces

(1) was is just fixed, as we trust nlohmann JSON to parse JSON
correctly, regardless of whitespace.

For (2), the existing URL was made a read-only test, since we very much
wish to continue parsing such invalid URLs directly. And then the
original read/write test was updated to properly percent-encode the
space, as the normal form should be.

src/libutil-tests/data/hash/blake3-base64.json

@@ -1,5 +1,5 @@
 {
-    "algorithm": "blake3",
-    "format": "base64",
-    "hash": "nnDuFEmWX7YtBJBAoe0G7Dd0MNpuwTFz58T//NKL6YA="
+  "algorithm": "blake3",
+  "format": "base64",
+  "hash": "nnDuFEmWX7YtBJBAoe0G7Dd0MNpuwTFz58T//NKL6YA="
 }

src/libutil-tests/data/hash/sha256-base64.json

@@ -1,5 +1,5 @@
 {
-    "algorithm": "sha256",
-    "format": "base64",
-    "hash": "8OTC92xYkW7CWPJGhRvqCR0U1CR6L8PhhpRGGxgW4Ts="
+  "algorithm": "sha256",
+  "format": "base64",
+  "hash": "8OTC92xYkW7CWPJGhRvqCR0U1CR6L8PhhpRGGxgW4Ts="
 }