Fix /css/css-flexbox/gap-007-* WPT tests (#36585)

These tests were locally failing for me, both on Servo and Gecko,
because these browsers choose a different default font than Blink,
making the flex items a bit bigger, and thus only fitting 2 items on the
first flex line, instead of 3 items.

Manually setting the `line-height` provides consistent results.

Testing: This doesn't affect CI, but fixes the local problem for me.

Signed-off-by: Oriol Brufau <obrufau@igalia.com>

tests/wpt/meta/MANIFEST.json

@@ -178949,7 +178949,7 @@
       ]
      ],
      "gap-007-lr.html": [
-      "9d0a3717816f1850c4cf0a666fe563c9309c7b33",
+      "567dad749dac46887703f44d648b12ef086ee623",
       [
        null,
        [
@@ -178962,7 +178962,7 @@
       ]
      ],
      "gap-007-ltr.html": [
-      "6b24fc44eea9e8cca2d62e550d45ebe395b050db",
+      "e0dfc48f788499db4be8090098abdbde2d274a69",
       [
        null,
        [
@@ -178975,7 +178975,7 @@
       ]
      ],
      "gap-007-rl.html": [
-      "1461416abe9557d55d5eed623b314f0260dc4ddf",
+      "dd0af032bad23d3c6217c58100cfc97a99933e87",
       [
        null,
        [
@@ -178988,7 +178988,7 @@
       ]
      ],
      "gap-007-rtl.html": [
-      "cff3440a760bbd45b32c87017f7c95099bf9ef1c",
+      "d043f03a8dabef7f3c2dd2609d0585919c772821",
       [
        null,
        [
@@ -421279,19 +421279,19 @@
       []
      ],
      "gap-007-lr-ref.html": [
-      "a3b461fa127e8f5ec25fc1732c04dcc0c544c1cb",
+      "6000deb624348e8abe31a16693f2d262837e1952",
       []
      ],
      "gap-007-ltr-ref.html": [
-      "13741eb9bd3de3221bd820d2ab1b65a4a15dc287",
+      "9972a8c5ccf9aef9d6ca3719b2c30902ecc17ad2",
       []
      ],
      "gap-007-rl-ref.html": [
-      "1ca3b5cea987b628dedf30b720504ed948c04d1d",
+      "9dfc1860b526556ca36eedd5bd33f1faa9dcd8a7",
       []
      ],
      "gap-007-rtl-ref.html": [
-      "9e329cc0a4401365cafebf36e738651989f8817f",
+      "8c16fbdb22c4c7f79c5dc28b829a4ced2554ab2a",
       []
      ],
      "gap-008-ltr-ref.html": [

tests/wpt/tests/css/css-flexbox/gap-007-lr-ref.html

@@ -14,6 +14,7 @@
     display: inline-flex;
     flex-direction: column;
     flex-wrap: wrap;
+    line-height: 18px;
   }
   section > div{
     background-color: grey;

tests/wpt/tests/css/css-flexbox/gap-007-lr.html

@@ -20,6 +20,7 @@
     flex-direction: column;
     flex-wrap: wrap;
     gap: 20px;
+    line-height: 18px;
   }
   section > div{
     background-color: grey;

tests/wpt/tests/css/css-flexbox/gap-007-ltr-ref.html

@@ -10,6 +10,7 @@
     display: inline-flex;
     flex-direction: column;
     flex-wrap: wrap;
+    line-height: 18px;
   }
   section > div{
     background-color: grey;

tests/wpt/tests/css/css-flexbox/gap-007-ltr.html

@@ -16,6 +16,7 @@
     flex-direction: column;
     flex-wrap: wrap;
     gap: 20px;
+    line-height: 18px;
   }
   section > div{
     background-color: grey;

tests/wpt/tests/css/css-flexbox/gap-007-rl-ref.html

@@ -14,6 +14,7 @@
     display: inline-flex;
     flex-direction: column;
     flex-wrap: wrap;
+    line-height: 18px;
   }
   section > div{
     background-color: grey;

tests/wpt/tests/css/css-flexbox/gap-007-rl.html

@@ -20,6 +20,7 @@
     flex-direction: column;
     flex-wrap: wrap;
     gap: 20px;
+    line-height: 18px;
   }
   section > div{
     background-color: grey;

tests/wpt/tests/css/css-flexbox/gap-007-rtl-ref.html

@@ -14,6 +14,7 @@
     display: inline-flex;
     flex-direction: column;
     flex-wrap: wrap;
+    line-height: 18px;
   }
   section > div{
     background-color: grey;

tests/wpt/tests/css/css-flexbox/gap-007-rtl.html

@@ -20,6 +20,7 @@
     flex-direction: column;
     flex-wrap: wrap;
     gap: 20px;
+    line-height: 18px;
   }
   section > div{
     background-color: grey;